Story highlights

Marco Rubio delivered a strong performance at the third GOP presidential debate

Jeb Bush to CNN: 'I'm not a performer'

Boulder, Colorado CNN —  

Marco Rubio shined on the debate stage Wednesday night — and the timing couldn’t have been better.

The Florida senator brought his A-game to the third Republican presidential debate of the campaign season, sharply attacking his rivals and the media, swatting away criticisms about his candidacy, and bringing a personal narrative to the prime-time event sponsored by CNBC.

Rubio’s strong performance could prove to be devastating for his one-time mentor, Jeb Bush. The former Florida governor and heir to a political dynasty came to Boulder badly in need of of a jolt to energize his stalled campaign. Though he appeared eager to attack Rubio, there were few breakout moments for Bush in a debate that will almost certainly stoke further concerns among his allies and donors about his long-term viability.

In the most intense exchange between the two men this cycle, Bush went after Rubio for missing votes in the Senate while running for the White House.

READ: Memorable quotes from the Republican debate

“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.

Rubio’s night

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.

READ: CNN’s Reality Check team inspects the claims

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