Ken Starr, a member of Trump's legal team who served as the independent counsel investigating former President Bill Clinton, lamented that the U.S. is now in the "age of impeachment."

"In this particular juncture in America's history, the Senate is being called to sit as the high court of impeachment all too frequently," Starr said. "Indeed, we are living in what I think can aptly be described as the 'age of impeachment.'"

"How did we get here, with presidential impeachment invoked frequently in its inherently destabilizing as well as acrimonious way?" he asked.

Starr said that "like war, impeachment is hell, or at least presidential impeachment is hell."

Sen. Lisa Murkowski, R-Alaska, said Monday that she was "still curious" to hear what ex-national security adviser John Bolton could say in testimony following the reported revelations in his upcoming book.

“I’ve said before I’m curious about what Ambassador Bolton might have to say," Murkowski said. "I’m still curious.”

In a written statement, Murkowski said, “I stated before that I was curious as to what John Bolton might have to say. From the outset, I’ve worked to ensure this trial would be fair and that members would have the opportunity to weigh in after its initial phase to determine if we need more information. I’ve also said there is an appropriate time for us to evaluate whether we need additional information —that time is almost here. I look forward to the White House wrapping up presentation of its case.”

Fellow moderate Republican Sens. Mitt Romney and Susan Collins also said Monday that the Bolton revelations strengthen the case for calling witnesses in Trump's impeachment trial. 

Sen. Josh Hawley, R-Mo., told "Fox & Friends" on Monday that if new testimony is approved, the Senate should hear from those Trump-sought witnesses too — something he tweeted about last week.

In the event additional witness testimony and documents are approved, Hawley said he has prepared subpoenas for testimony and documents from the Bidens, the whistleblower who alerted Congress to Trump's Ukraine dealings and lead House manager Adam Schiff, among others. His office said the subpoenas would also include documents and testimony from and intelligence community Inspector General Michael Atkinson.

Of Bolton's book, Hawley said "it's certainly going to sell a lot of" copies.

"Listen, I can't tell from the New York Times report what is actually being reported here," he said. "I can't tell if this is something new. I can't tell if they've actually seen the manuscript. It's all a bunch of hearsay and clearly it's an attempt to try to influence the course of the trial."

President Donald Trump told reporters at the White House that "nothing was ever said" to his former national security adviser, John Bolton, who reportedly alleges in an unpublished book that Trump told him he would continue to withhold aid to Ukraine to pressure its leaders to announce investigations into Democrats, including the Bidens.

"Well, I haven't seen a manuscript, but I can tell you, nothing was ever said to John Bolton. But I have not seen a manuscript. I guess he's writing a book. I have not seen it."

Trump tweeted earlier Monday, "I NEVER told John Bolton that the aid to Ukraine was tied to investigations into Democrats, including the Bidens," Trump wrote. "In fact, he never complained about this at the time of his very public termination. If John Bolton said this, it was only to sell a book."

"The Democrat controlled House never even asked John Bolton to testify," the president added, though the House asked but did not subpoena Bolton for testimony. "It is up to them, not up to the Senate!"

Sen. Lindsey Graham, R-S.C., said Monday that he is interested in subpoenaing John Bolton's manuscript for his upcoming book — in which the former national security adviser reportedly claims President Trump linked Ukrainian aid to the investigations of Democrats in an August 2019 discussion.

"I want to know what's in the manuscript," Graham said. "Yeah, I think that’s important."

Graham said Bolton's claims are "probably" not going to "change" his view of Trump's innocence, "but I’ll determine whether or not it’s relevant.'

Asked if he trusts Bolton, Graham said, "I don’t know if I trust anybody right now."

"He may be a relevant witness, but I’ve also said I want to comply with reasonable requests by the president about the Bidens and their involvement with Burisma," he said, adding, "We’re not going to get part of it, we’re going to get all of it."

Graham declined to commit to voting in favor of witnesses and documents.

Sens. Mike Braun, R-Ind., and John Barrasso, R-Wyo., told reporters at a Monday news conference that they weren't terribly concerned with Bolton's reported claims and pointed to the president having denied making such comments to his then-national security adviser.

But Braun said the revelations "may move the needle" toward a vote on witnesses in Trump's impeachment trial.

The latest reports on Bolton's book will "change the decibel level and the intensity of which we talk about witnesses," Braun said, earlier pointing to the president's denial.

Barrasso called the reporting a "so-called blockbuster" and said it was more a "story about selective leaks." He insisted the president did not engage in a quid pro quo with regard to Ukraine, said there is "nothing new here," and compared the revelations to reports from Supreme Court Justice Brett Kavanaugh's confirmation hearing.

Their comments came after Romney and Collins said the revelations from the soon-to-be-released book strengthened the case for witnesses, with Romney saying it was "increasingly likely" enough Republicans will vote in favor of new testimony.

A source familiar with the matter says the president's defense team was largely blindsided by The New York Times report on the Bolton book, as were members of Congress. Note that National Security Council spokesman John Ullyot said in a statement, "No White House personnel outside NSC have reviewed the manuscript." It's possible that could be an attempt to absolve Cipollone and his team from blowback if they had known and didn't share.

What's the thinking about witnesses now from that end of Pennsylvania Avenue

An acknowledgment that this increases the pressure on the Senate to call witnesses. So what will the defense team do if in fact senators vote to call witnesses? Sources have repeatedly said they're prepared for all contingencies — including that one.

Sen. Susan Collins, R-Maine, tweeted out a statement saying the "reports about John Bolton's book strengthen the case for witnesses and have prompted a number of conversations among my colleagues."

The statement also maintained, "I've always said that I was likely to vote to call witnesses, just as I did in the 1999 Clinton trial."

Another moderate Republican senator, Mitt Romney of Utah, said earlier Monday that it's "increasingly likely" there will be enough Republican senators to vote in favor of calling witnesses in the president's ongoing trial.

"I think, with the story that came out yesterday, it's increasingly apparent that it would be important to hear from John Bolton," Romney told reporters in brief comments.

Read the full story.

Senate Minority Leader Chuck Schumer, D-N.Y., called the revelations from a soon-to-be released book by former national security adviser John Bolton “stunning” on Monday and implored Senate Republicans to vote to call Bolton and others to testify.

“It goes right to the heart of the charges against the president,” Schumer said, referring to The New York Times report on a manuscript of Bolton’s book.

“Bolton essentially confirms the president committed the offenses charged in the first article of impeachment,” Schumer said. “He is ready and willing to testify. How can Senate Republicans not vote to call that witness and request his documents?”

“It’s up to four Senate Republicans — just four Senate Republicans,” Schumer said. 

The minority leader added that if Senate Republicans “are not going to vote to call Bolton” and acting White House chief of staff Mick Mulvaney, “they’re going to be part of the cover-up.”

What's today's overriding headline?

The Bolton book. You saw the president’s tweet-denials overnight and this morning of the allegations Bolton reportedly makes. Pompeo, Mulvaney and Barr all mentioned as well. Hill team will have best guidance on what they’re hearing from Senate Republicans on whether this will be a game-changer for the impeachment trial witness vote, but the White House is keeping a close eye.

What to expect from defense team today

The “big guns,” so to speak, and the argument that the president’s conduct did not rise to the constitutional level of removal. This will be Ken Starr and Alan Dershowitz’s areas of presentation, among others. You will likely hear more about the Bidens. And it’ll go longer than Saturday’s short session, though TBD on whether the defense team will choose to finish up Tuesday or not.

Meanwhile, at the White House ...

The president is meeting with both Benjamin Netanyahu and his political rival Benny Gantz (separately!) today, with his Middle East peace plan on the agenda.

Sen. Mitt Romney, R-Utah, said Monday it's "increasingly likely" that there will be enough Republicans to vote in favor of calling witnesses in President Donald Trump's Senate trial in the wake of the major revelations from a soon-to-be released book from former national security adviser John Bolton.

At least four Republicans would need to vote alongside all Democratic senators in order to secure new testimony.

Romney, Susan Collins, R-Maine, Lisa Murkowski, R-Alaska, and Lamar Alexander, R-Tenn., are considered to be most likely Republicans to vote in favor of witnesses.

Other Republicans cast doubt on the bombshell report that Bolton alleges in his book that Trump directly linked the withheld Ukrainian military aid and his push for investigations into Democrats. And they also said that if the Senate now votes to hear from witnesses such as Bolton, senators better allow for Trump's preferred witnesses to be called to testify as well.

Read the full story.

President Donald Trump tweeted on Monday that he "NEVER" told former national security adviser John Bolton that the hold on nearly $400 million in military aid was tied to investigations of Democrats after it was reported Bolton insisted as much in an upcoming book.

"I NEVER told John Bolton that the aid to Ukraine was tied to investigations into Democrats, including the Bidens," Trump wrote. "In fact, he never complained about this at the time of his very public termination. If John Bolton said this, it was only to sell a book."

"The Democrat controlled House never even asked John Bolton to testify," Trump added, though the House asked but did not subpoena Bolton for testimony. "It is up to them, not up to the Senate!"

According to a manuscript of Bolton's book, obtained by The New York Times and not seen by NBC News, Trump told Bolton in August that nearly $400 million in aid to Ukraine would not be released until it provided all of the information it had in connection to the investigations of Democrats that the president sought. One month earlier, Trump asked Ukrainian President Volodymyr Zelenskiy to investigate former Vice President Joe Biden, his son Hunter Biden and Democrats.

Read the full story.

Democrats stepped up their calls Sunday night for former national security adviser John Bolton to testify at President Donald Trump's impeachment trial after an explosive report alleged that in his unpublished book, he said Trump personally tied aid for Ukraine to an investigation of the Bidens — an account that conflicts with the president's.

"John Bolton has the evidence," Senate Minority Leader Chuck Schumer, D-N.Y., tweeted.

According to the manuscript, as reported by The New York Times on Sunday night, Trump told Bolton that nearly $400 million in aid to Ukraine would not be released until it offered assistance with investigations of Democratic targets, including former Vice President Joe Biden and his son, Hunter.

NBC News has not seen a copy of the manuscript or verified the report, which cited multiple sources familiar with Bolton's account.

Read the full story.

The formal documents on the impeachment of President Donald Trump ring with charges of greed and betrayal. But presidents don’t get impeached only because — or even mainly because — they’re egregiously corrupt. They get impeached because they don’t know how to get what they want without violating laws, rules and norms in order to do it.

With Trump, we have a president who regularly demonstrates he has no knowledge of the intricacies of how the government works — and didn’t think he had to learn.

We are watching this play out now in his Senate impeachment trial. The House managers have been presenting their case of alleged corruption. But they’ve also exposed the sheer dumbness of Trump's efforts to get the Ukrainians to produce anti-Joe Biden campaign fodder for him. Even with administration partisans, including the attorney general, in control of the Justice Department, Trump couldn't figure out how to use the regular levers of government to get what he wanted. He had to resort to his private lawyer Rudy Giuliani and his "associates." That was a potentially disastrous choice — and the disaster has now materialized.Read the full opinion piece.

The Senate’s trial into the charges against Trump began last week, marking it only the third time in American history that a president has been impeached.

While the trial has been live on TV and the internet and made the front pages of newspapers across the country, the public’s interest has waxed and waned. Some people say they believe the outcome of the trial has been predetermined, causing their interest to fade. Others have remained engaged, following every detail, while still others have pulled away completely.

Full story here.

Sen. Mike Braun, R-Ind., said Sunday that he hopes President Donald Trump's impeachment "will be instructive" to the president so that he is more "careful" about his actions going forward.

"Hopefully it will be instructive to where ... I think he'll put two and two together," Braun told NBC's "Meet the Press." "In this case, he was taken to the carpet."

"I think he’ll be instructed by what has occurred here and certainly any individual would want to avoid whatever might need to be modified to go through this again because the threat is already been out there that 'we might find something else to impeach you on," Braun added, pointing to Democrats. "Which I think is a mistake because I think we need to get back to what most Americans are interested in, the agenda."

The Hoosier State senator said the process "ought to be instructive to anyone here that if you're pushing the envelope or doing things that may not feel right, let alone be right, you better be careful."

Read more here.

As part of President Donald Trump's impeachment defense team, Alan Dershowitz is expected to argue on behalf of the president this week. But in a 2016 book he authored, the famed defense attorney called Trump a “destabilizing and unpredictable candidate,” warning that the then-presidential candidate “openly embraces fringe conspiracy theories peddled by extremists.”

Dershowitz wrote those statements in his book titled “Electile Dysfunction: A Guide for Unaroused Voters.” As part of Trump’s defense team, Dershowitz will outline the “foundations of what it means to rise to the level of what is impeachable and what is not,” Jay Sekulow, one of Trump’s lead impeachment lawyers, said last week on Fox News.

In a phone call with NBC about his comments in the 2016 book, Dershowitz clarified his views about the president. “I was campaigning for Hillary Clinton at the time. I hadn’t really ever met President Trump and it was just typical campaign rhetoric," he said. "I would not repeat that characterization today having met him.”

Read more here.

House impeachment managers presented their case against the president over three days this week, and Trump's legal team launched its defense in a short session on Saturday. If you're just catching up, here's what you missed:

• Senate passes McConnell impeachment rules after nearly 13 hours of debate.
• What happened on day two.
• What happened on day three.
• What happened on day four.
• What happened on day five.

Democrats believe they have more than a smoking gun in President Donald Trump's impeachment trial. They have a trigger man, they have a motive and they have a record of the key moment.

What they would like more of — but do not believe would be necessary in a jury trial — is access to documents they know exist and witnesses close to Trump whom they believe would further support the case for removing him from office.

"This is airtight," said a person familiar with the prosecution, who noted that all of the witness testimony obtained during the House investigation corroborated a long campaign by top Trump lieutenants to effect the president's Ukraine plan. "What [we] don't have is someone saying, 'I helped orchestrate that monthslong effort.'"

The weaknesses of the case can be found in the political strength of a president whose fellow Republicans are expected to vote nearly or fully in lockstep to keep him in office, regardless of what they think of his actions — a handful will say they were imperfect, and others will say they see nothing wrong with what he did — and in his ability to use executive fiat to prevent prosecutors from obtaining evidence from his closest circle of advisers and documents housed at various federal agencies.

Read more here.