Here's what we learned from the fourth Republican presidential debate, which was held in Milwaukee and hosted by Fox Business Network and the Wall Street Journal on Tuesday.
1. The candidates lagging behind in polls are ready to attack
In the first three debates, the confrontations were often between the moderators and the candidates. Not this time. Both ex-Florida Gov. Jeb Bush and Ohio Gov. John Kasich, stuck in single digits in most polls, blasted Donald Trump's proposal to force the estimated 11 undocumented immigrants currently in the United States to leave the country. Kasich called Trump's plan a "silly argument," while Bush said "it's not embracing American values" and warned "they are doing high-fives in the Clinton campaign" at the discussion of Trump's idea.
Trump, at the top of most polls, strongly defended his immigration views. He argued that allowing the undocumented immigrants to remain in the country is "very unfair" to people who have followed the traditional process to enter the United States.
Kentucky Sen. Rand Paul, also well behind, focused on Florida Sen. Marco Rubio, who has received a number of endorsements in the last few weeks from GOP elected officials and has become the favorite of many in the Republican establishment.
"How can you be conservative and be for unlimited military spending?" Paul said, attacking Rubio's proposals to increase defense spending.
Rubio, like Trump, gave little ground, arguing nothing was more important than defending the country and calling Paul a "committed isolationist."
The policy differences were not new. Bush and Kasich are more liberal on immigration than Trump, and Paul is more reluctant to use military force abroad than any of the other GOP 2016 candidates.
But it suggests that the campaign is entering a new phrase. Trump and neurosurgeon Ben Carson have been leading polls for weeks, and Rubio is consolidating support from Republicans who want a more traditional candidate. Their rivals seem to understand they need to attack these candidates more sharply to defeat them
2. Ben Carson got past biographical questions, but struggled on policy
Fox Business moderators had promised the debate would be more focused on policy issues, and that benefited Carson. Amid a series of controversies about whether he has made up or exaggerated parts of his biography, the surgeon was only asked a single question about his past.
"I have no problem with being vetted. I do have a problem being lied about," Carson said.
Carson said he was being truthful in describing how military officials had suggested he would be admitted to West Point if he had applied when he was graduating from high school.
"People who know me know that I'm an honest person," Carson said.
At the same time, as in previous debates, Carson largely gave vague answers when asked about his policies on taxes or how to fight ISIS. So far, his supporters, particularly evangelical Christians, have said they like Carson's slow-talking, deliberate approach.
But if Carson wants to win the support of key Republican donors and elected officials, his performance on Tuesday is unlikely to help. Other candidates like Kasich, Texas Sen. Ted Cruz and even Trump seem more versed and conversant on key policy issues.
3. The immigration debate is heating up
Beyond biographical factors, the biggest difference between the Republican candidates is on immigration policy. And that contrast emerged clearly on Tuesday, with Cruz and Trump sharply differing with Bush and Kasich. Both Bush and Kasich support creating some kind of path to legalization for the undocumented, which Trump and Cruz strongly oppose.
"If Republicans join Democrats as the party of amnesty, we will lose," said Cruz.
Rubio opted not to enter that exchange. But Cruz and Trump are increasingly targeting the Florida senator, who was the co-author of an immigration bill in the Senate that would have created a pathway to citizenship for the undocumented. That issue looms as one of the biggest challengers for Rubio, who is now considered by many to be the favorite to win the GOP nomination.
4. Trump is behaving like someone who thinks that he could be president
In the early debates, Trump was full of bombast, mocking the other candidates for their looks and low poll numbers. But on Tuesday, the real estate mogul largely stuck to blasting President Obama and Hillary Clinton and taking traditional GOP positions on most issues.
Early in the race, Trump seemed a very unusual Republican presidential candidate. But he seems to fit more with the party now. He and Cruz agreed on immigration, and Trump joined the other Republicans in opposing an increase to the minimum wage. Carson, who didn't speak for large stretches of time in the debate, is more of an outlier in terms of style than Trump.
Trump of course did specifically criticize Carly Fiorina, asking "why does she keep interrupting people?"
5. Rubio emerged unscathed
With the Republican establishment looking for a candidate who is not Carson or Trump, Rubio has emerged as the most logical figure. And he did nothing on Tuesday to stop his momentum. He defended his policy positions from Paul's attacks, and Bush, who had been expected to criticize Rubio, opted not to.
6. The Republicans browbeat the media and it worked
Republicans had been furious about the recent debate hosted by CNBC, which they felt both did not give the candidates enough time to speak about issues and included too many "gotcha" questions. The Fox Business News moderators seemed to hear the complaints. Previous debates had included many questions that required a candidate to respond to a criticism from one of his rivals.
In this session, the questions were mostly simply asking the candidates to explain their plans or react to news events, like a federal circuit court ruling this week that Obama's executive actions on immigration were illegal. In addition, there were only eight, instead of 10 candidates on stage, so each of them had more time to speak.
"Congrats to @Reince @SeanCairncross and the RNC team for a fine debate. @FoxBusiness deserves great credit for substantive Qs," wrote Josh Holmes, a top adviser to U.S. Senate Majority Leader Mitch McConnell, in a Twitter message after the debate.
Reince Priebus is the chairman of the Republican National Committee, and Sean Cairncross had been tapped by Priebus to oversee the debates after criticism of the CNBC one.