Two key GOP swing votes, Sens. Lamar Alexander of Tennessee and Lisa Murkowski of Alaska, signed on to a question from Sen. Lindsey Graham and other Republicans asking whether John Bolton's testimony would even make a difference.

"Assuming, for arguments' sake, Bolton were to testify in the light most favorable to the allegations contained in the articles of impeachment, isn't it true that the allegations still would not rise to the level of an impeachable offense and that therefore, for this and other reasons, his testimony would add nothing to this case?" the group of GOP senators asked. 

Trump's defense team responded by arguing that Bolton's testimony wouldn't matter because nothing the former national security adviser says Trump did is impeachable.

"Assuming for the sake of argument that ambassador Bolton would come and testify the way "The New York Times" article alleges, the way his book describes the conversation," Philbin said, referring to an unpublished Bolton book in which he alleged that Trump linked aid for Ukraine to Biden investigations.

"Then it is correct that even if that happened, even if he gave that testimony, the articles of impeachment still wouldn't rise to an impeachable offense."

He added, "Even if everything you allege is true, even if John Bolton would say it's true, that is not an impeachable offense under the constitutional standard. Because the way you've tried to define the constitutional standard, this theory of abuse of power is far too malleable."

Schiff fired back that Bolton's testimony would undeniably be pertinent because it would underscore the House's evidence alleging a link between the withholding of military aid and Ukraine investigating the Bidens. 

"The truth is staring us in the eyes. We know why they don't want John Bolton to testify. It's not because we don't really know what happened here. They just don't want the American people to hear it in all of its ugly graphic detail," Schiff said. 

Sens. Gary Peters, D-Mich., and John Cornyn, R-Texas, asked both sides whether the verdict of the impeachment trial could alter the balance of power between Congress and the White House. 

Schiff said that if Trump is acquitted it will "irrevocably" alter the balance of power between the two branches because there would no case in which the president can be held accountable by Congress, making impeachment power a "nullity." 

"Article II will really mean what the president says it means, which is he can do whatever he wants. So yes, the stakes are big here," Schiff said. "Article II goes to whether our oversight power, particularly in a case of an investigation president's own wrongdoing, continues to have any weight whether the impeachment power itself is now a nullity." 

However, Cipollone argued that an acquittal that would not alter the separation of powers because the impeachment was "purely partisan" and it would reaffirm the powers of the president and the role of Congress. 

"The final judgment of acquittal would be the best thing for our country and would send a great message that will actually help in our separation of powers," he said.

"Here is why: As I have said repeatedly, and according to the standard articulated so well during the Clinton impeachment - what are we dealing with here? We're dealing with a purely partisan impeachment with bipartisan opposition, no crime, no violation of law, in an election year. Never happened before."

Sen. Lisa Murkowski, R-Alaska, considered a key swing vote on whether to call additional witnesses in Trump's Senate trial, signaled in a question Thursday night that she's leaning in favor of hearing from people like former national security adviser John Bolton.

Murkowski directed her question to Trump’s legal team, reminding them that they "explained that Ambassador Sondland and Senator Johnson both said the president explicitly denied that he was looking for a quid pro quo with Ukraine."

She continued: "The reporting on Ambassador Bolton's book suggests the president told Bolton directly that the aid would not be released until Ukraine announced the investigations the president desired. This dispute about material facts weighs in favor of a calling additional witnesses with direct knowledge. Why should this body not call Ambassador Bolton?"

According to a manuscript of Bolton's book reported on 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.

Patrick Philbin, a White House lawyer who is on Trump's defense team, said House Democrats brought the articles of impeachment to the Senate “half baked, not finished." He said that it would be “damaging for the future of this institution” if the Senate calls additional witnesses. 

Bolton, who refused a request to cooperate in the House impeachment inquiry but was not subpoenaed, has said he'd testify before the Senate if subpoenaed to do so. 

Sen. Lamar Alexander, one of the few Republicans considering voting yes on witnesses, said that he will announce his decision tonight after the question-and-answer session wraps up.

Alexander has been consistent in his desire to announce his decision on witnesses after arguments from both sides and Q&A is finished.

It's unclear how he will vote.

The Senate is back in session after an hour-long dinner break. The first question is about partisan impeachment.

Sen. Lamar Alexander’s first question during the Senate trial came Thursday evening when he, along with two other senators, asked the House managers to compare the bipartisanship in the Nixon, Clinton and Trump impeachment proceedings. 

Alexander asked "specifically how bipartisan was the vote in the House of Representatives to authorize and direct the House Committees to begin formal impeachment inquiries for each of the three presidents?"

The question was notable because Alexander, who’re retiring from Congress at the end of the year, is considered a swing vote on whether witnesses should be called during the trial.

It could be a sign that Alexander and other GOP senators were frustrated that the impeachment vote in the House against Trump was not bipartisan. 

Rep. Zoe Lofgren, D-Calif., was a staffer on the House Judiciary Committee during the Nixon impeachment proceedings and a member of the committee during both the Clinton and Trump impeachments. 

"In the Nixon impeachment we look back and we think about the vote on the House Judiciary Committee that ended up bipartisan but it didn't start that way," she said. "When it came to the Clinton impeachment. That was, again, It started out along very partisan lines. And it ended along partisan lines."

In response, Senate Majority Leader Mitch McConnell, R-Ky., asked the White House legal team to respond to Alexander’s question. 

The trial has begun a 45 minute break. There have been 55 questions Thursday so far, 27 from Democrats, 26 from Republicans, and two bipartisan.