Republican debate: Marco Rubio hobbles Jeb Bush

"Marco, when you signed up for this, this was a six-year term and you should be showing up to work," Bush said. "What is this, like a French work week?"

Bush then delivered another punch: "Just resign and let someone else take the job."

But Rubio fired back, saying Bush never took issue with Sen. John McCain missing votes when he was running for president.

"The only reason you're doing it now is because we're running for the same position," the senator said.

The back-and-forth underscored the competing rivalries that are simmering beneath a presidential race that has so far been dominated by Donald Trump and Ben Carson's insurgent campaigns. Bush's decision to go after Rubio for his work ethic in the Senate showed that he believes Rubio is blocking his lane. Bush, struggling to break through to the top of the GOP pack, was clearly trying to deliver the attack of the evening -- but Rubio quickly and effectively counterpunched.

After the debate, Bush told CNN's Dana Bash he wasn't "frustrated" by his performance.

"I'm running with heart," Bush said. "I'm not a performer. If you're looking for an entertainer in chief, I'm probably not the guy."

The debate offered candidates an opportunity to spar over everything from taxes to the Federal Reserve and fantasy football. And it provided a moment of unity as many candidates blasted the CNBC moderators for the tone of their questioning.

Still, it was Rubio's night. His confidence was all the more notable next to Trump's uncharacteristically low-key performance. The billionaire businessman has dominated the GOP race since launching his White House bid this summer, but has finally started to show signs of vulnerability as he's lost ground to Ben Carson in Iowa polls and a key national survey.

Rubio also appeared to contrast himself from some of his rivals on stage, including Bush and Trump, without actually naming them.

Asked to address some of his personal financial troubles — and what those challenges say about Rubio's ability to manage the country's finances — he offered a seamless response about his humble upbringing.

"Here's the truth. I didn't inherit any money," Rubio said, before explaining how his parents didn't save enough money to send him to school and how he and his wife have had to work to provide for their four children.

"This debate needs to be about the men and women across the country that are struggling across the country on a daily basis," he added.

Trump and Carson's underwhelming performance here bodes well for Ted Cruz. The Texas senator has been waiting behind his two anti-establishment rivals to have his own moment in the race, and also showed off his strongest debate performance yet in Boulder.

His most explosive response of the night came in the form of pounding the media. The senator was asked whether his opposition to a budget agreement reached in Washington this week revealed that he is "not the kind of problem solver Americans voters want."

Cruz unleashed a lengthy indictment of the media.

"The questions that have been asked so far in this debate illustrate why the American people don't trust the media," he said. "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?'"

As the crowd roared, Cruz added: "How about talking about the substantive issues the people care about?"

The Republican National Committee joined in on the criticism.

"I'm disappointed in CNBC. You know, I thought they would bring forward a pretty fair forum here tonight," RNC Chairman Reince Priebus told Bash in the spin room following the debate. "But I think it was one gotcha question, one personal low blow after another."

The debate was Carson's first since rising to the top of the polls and it tested the political newcomer on his preparedness to be on the national stage. True to his repeated promises to remain true to his character, the retired neurosurgeon declined to go after his rivals.

Carson has made several controversial comments as a White House candidate, including about homosexuality and Muslims, and reiterated on the debate stage that he will not change his public rhetoric.

He said the idea that a person who believes marriage is between a man and a woman is a homophobe was "one of the myths the left perpetrates on our society."

"That's what PC culture is all about," Carson said. "It's destroying this nation."

Ahead of the prime-time debate, the lowest-ranking GOP candidates -- Rick Santorum, Bobby Jindal, George Pataki and Lindsey Graham -- battled it out at the Coors Events Center at the University of Colorado.

For all four men, the pressure's on to raise their national poll numbers to qualify for the main debate stage hosted by Fox Business Network on November 10. The so-called undercard candidates offered their views on a budget agreement passed by the House earlier in the day that would lift the debt ceiling and avert a government shutdown.

Graham, a national security hawk, called President Barack Obama an "incompetent commander in chief" but said he approved provisions in the deal that would add billions of dollars to the Defense Department's budget. Pataki, who was equally critical and accused Obama of holding the military "hostage," said though he believed it was a "bad deal," he would sign it in order to "protect our military."

Jindal was critical of the agreement, but said closing the government over the budget deal was a "false choice."

Graham was put on the spot for several policy stances unpopular among conservatives, including believing climate change is real, being willing to accept tax increases and supporting a path to citizenship for those who are in the country illegally.

"I'm not a scientist and I've got the grades to prove it," Graham said, drawing laughter from the room. But the majority of scientists, he added, "are telling me that greenhouse gas effects are real, that we're heating up our planet."

Regarding immigration, Graham said he doesn't believe in mass deportation, but instead wants to fix the problem by, in part, securing the country's borders. "I want to talk about fixing the problem."