The Times reporter Michael D. Shear analyzes key moments from the third Republican presidential debate in Boulder, Colo., on Wednesday. It included sharp early exchanges, including one between Jeb Bush and Marco Rubio.  Senator Marco Rubio of Florida coolly rebuffed attacks from his onetime mentor, Jeb Bush, and Senator Ted Cruz of Texas emerged as a champion of social conservatives at Wednesday’s Republican presidential debate, as both men found their voices after months of lower-key performances.  Testiness and sharpened jabs infused the night as struggling candidates like Mr. Bush, the former governor of Florida, and Gov. John R. Kasich of Ohio ripped into their less experienced rivals and tried to portray them as unqualified for the White House.  The free-for-all of verbal assaults reflected the new volatility in a race that Donald J. Trump dominated for months. It appears to be shifting in favor of candidates like Mr. Rubio and Ben Carson as the first nominating contests near and voters start paying closer attention to the field.  Mr. Rubio, a first-term senator, had the best night of his campaign, showing the political talent that many insiders had long seen in him. He and Mr. Carson, a retired neurosurgeon now leading in several polls, faced the toughest questions but emerged largely unscathed, with Mr. Rubio in particular winning strong applause from the audience at the University of Colorado Boulder for his confident performance and deft counterpunches.  Mr. Bush, under great pressure to have a strong debate that would reassure his supporters and change the trajectory of his struggling campaign, had another lackluster night, raising the possibility that uncommitted donors will write him off and embrace candidates like Mr. Rubio.  Frustration permeated the Bush camp: Danny Diaz, Mr. Bush’s campaign manager, told reporters that he had complained to CNBC, the cable network hosting the debate, that Mr. Bush was receiving too few questions from the moderators.  Mr. Kasich made a strong impression by showing new aggressiveness from the debate’s first moments, taking on Mr. Trump and Mr. Carson in hopes of improving his dismal standing in voter surveys. Mr. Trump, bent on recapturing his lead in the polls from Mr. Carson, was more restrained in his mockery of his rivals than in the previous two debates, and even faded into the background for long stretches.  In a shift from the first two debates, Jeb Bush trailed all candidates in speaking time on Wednesday, while Marco Rubio and Carly Fiorina spoke the most.  It was a night that saw a reversal of fortunes: Mr. Trump and Mr. Bush have been in the campaign spotlight for months as they jousted with each other and asserted their financial dominance in the race, but both were overshadowed Wednesday by the commanding performances of Mr. Rubio and Mr. Cruz.  Mr. Rubio, an ally of Mr. Bush when he was governor of Florida and Mr. Rubio was the State House speaker, found himself under sharp attack over his reputation for chronic absenteeism in Washington. Mr. Rubio has missed more votes than any other senator this year. Mr. Bush, who has fared poorly with voters despite months of campaigning and heavy spending, blasted Mr. Rubio over his work ethic — a striking moment in the ongoing fraying of their friendship as they compete for support from moderate Republicans in Florida.  “Marco, when you signed up for this, this was a six-year term — you should be showing up to work,” Mr. Bush said. “I mean, literally, the Senate, what is it, like a French workweek? You get like three days where you have to show up?”  Mr. Rubio hit back forcefully, noting that Mr. Bush has said he is modeling his campaign after Senator John McCain’s in 2008, and that Mr. McCain missed many votes in the chamber during that run. And he attributed the criticism to the fact that Mr. Bush is struggling in the polls.  “The only reason you’re doing it now is because we’re running for the same position and someone has convinced you that attacking me is going to help you,” Mr. Rubio said.  Mr. Cruz also stood out far more than he had in the first two debates, reminding viewers of his fights against Republican leaders and blistering the news media in a fashion that delighted the crowd. He made a strong case for why he could emerge as the candidate of choice for evangelical voters, a vital Republican bloc.  “My mom, who’s here today, was a single mom when my father left us when I was 3 years old,” Mr. Cruz said. “Now, thank God, my father was invited to a Bible study and became born again, and he came back to my mom and me, and we were raised together. But I — the struggle of single moms is extraordinary.”  Mr. Bush, whose mild-mannered, pragmatic persona and family lineage have turned off many voters, drew little applause as he dwelled mostly on policy and did little to undercut his chief antagonist in the race, Mr. Trump, who has repeatedly belittled him. He did not directly take on Mr. Trump, saying obliquely and in somewhat rambling language, “It troubles me that people are rewarded for tearing down our country.”  “It’s never been that way in American politics before,” Mr. Bush continued. “I just don’t believe that this country’s days are going to be deeply — you know, going down. I think we’re on the verge of the greatest time, and I want to fix the things to let people rise up.”  The most interesting moments during the third Republican presidential candidate debate in Boulder, Colo.  Mr. Bush receded after his unsuccessful skirmish with Mr. Rubio, failing to turn in the sort of bravura performance his supporters had hoped for.  Even when Mr. Bush boasted lightheartedly about his undefeated fantasy football record this year, the line was hurled back in his face. “We have $19 trillion in debt, we have people out of work, we have ISIS and Al Qaeda attacking us, and we’re talking about fantasy football?” Gov. Chris Christie of New Jersey demanded, turning his fire on Mr. Bush and the moderators, who had asked about regulation of the fantasy sports industry.  While the debate was ostensibly dedicated to the economy, the fluid dynamics of the Republican race — with new polls showing Mr. Carson in the lead in Iowa and nationally, and Mr. Trump and especially Mr. Bush in decline — drove the candidates to seek moments that would emphasize their credibility and electability and resonate with conservative voters who are dissatisfied with Washington and politics as usual.  Mr. Kasich was more impassioned than Mr. Bush in his attacks, which began when Mr. Kasich ignored the debate’s first question, about each candidate’s weaknesses. Instead, he made implicit criticisms of Mr. Trump and other Republicans as unprepared for the presidency or wrongheaded on issues, referring, for example, to Mr. Trump’s strong support for deporting unauthorized immigrants.  “My great concern is that we are on the verge, perhaps, of picking someone who cannot do this job,” Mr. Kasich said. “We need somebody who can lead.”  Mr. Trump ripped into Mr. Kasich for turning negative against his rivals. “He was such a nice guy, and he said, ‘Oh, I’m never going to attack,’ ” Mr. Trump said.  “But then his poll numbers tanked. He has got — that is why he is on the end,” he added, referring to Mr. Kasich’s placement on the debate stage. “And he got nasty. And he got nasty. So you know what? You can have him.”  Several candidates faced tough questioning about their financial policies. Mr. Carson defended his plan to radically overhaul the tax system: He has said he would take inspiration from God and push for a “proportional tax system” based on tithing, in which people would pay the same percentage — close to 15 percent — of income in taxes, while deductions and loopholes would be eliminated.  When a moderator insisted that his plan would leave the government with a trillion-dollar revenue hole, Mr. Carson pushed back.  “That’s not true,” he said. “It works out very well.”  Mr. Kasich was quick to dispute Mr. Carson. “This is the fantasy that I talked about in the beginning,” he said of Mr. Carson’s tax ideas. “These plans would put us trillions and trillions of dollars in debt.”  Mr. Rubio seemed to get the better of Mr. Bush again when they discussed their own economic plans in close succession. While Mr. Bush spoke wonkishly of “the code” and “regulatory cost,” Mr. Rubio cast the issue in personal terms, talking about how a tax overhaul would affect “the guy that does my dry cleaning.”  Mr. Rubio also faced questions about money — in his case, about his management of his personal finances: He has acknowledged mistakes such as using personal credit cards to pay for his campaigns, and using a Florida Republican Party credit card to pay for a paving project at his home and for travel to a family reunion. He responded by implicitly swiping at Mr. Bush and Mr. Trump, who benefited from being the scions of wealthy fathers.  “I didn’t inherit any money — my dad was a bartender and my mother was a maid,” Mr. Rubio said. “I’m not worried about my finances; I’m worried about the finances of everyday Americans who today are struggling in an economy that is not producing good-paying jobs.”  Again and again, Mr. Rubio repelled attacks from his opponents and difficult questions from the moderators with ready responses that turned issues back on his interlocutors and prompted loud applause from the crowd.  He defended himself by attacking a Florida newspaper that had called for his resignation on Wednesday — and, more broadly, by castigating the news media, a favorite target of Republicans.  Mr. Cruz picked up where Mr. Rubio left off and won his own round of booming applause for attacking the CNBC moderators, saying they were focused on provoking fights rather than examining policy issues.  “This is not a cage match. And you look at the questions — Donald Trump, are you a comic book villain? Ben Carson, can you do math? John Kasich, will you insult two people over here? Marco Rubio, why don’t you resign? Jeb Bush, why have your numbers fallen?” Mr. Cruz said. “How about talking about the substantive issues people care about?”  Mr. Rubio appealed to the audience not only with his attacks on the news media, but also by taking aim at Hillary Rodham Clinton, the leading Democratic presidential candidate.  “It was the week she got exposed as a liar,” he said, referring to Mrs. Clinton’s testimony last Thursday before a congressional panel investigating the 2012 attacks on the American Mission in Benghazi, Libya.  Another candidate, Carly Fiorina, the former Hewlett-Packard chief, who benefited from her debate showing last month, did not make the same impact on Wednesday. She echoed Mr. Rubio’s comments about Mrs. Clinton in her concluding statement, looking into the camera and telling viewers: “In your heart of hearts, you cannot wait to see a debate between Carly Fiorina and Hillary Clinton.”
In a change from previous debates, the candidates dove into the gritty details of economic plans and entitlement programs, but exchanges grew contentious and political friendships were strained.