BOULDER, Colorado (CNN) -- Jeb Bush and Marco Rubio are done playing nice.
Sparks flew between the two Republican presidential candidates tonight at the third GOP presidential debate of the campaign season.
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 --- an issue that critics have seized upon.
"Marco, when you signed up for this, this was a six-year term and you should be showing up to work," Bush said at the debate sponsored by CNBC. "What is this, like a French work week?"
Bush then delivered another punch: "Just resign and let someone else take the job."
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 campaign that has been dominated by Donald Trump. Bush's decision to go after Rubio for his work ethic in the Senate shows that he believes Rubio is blocking his lane. Bush, struggling to break through to the top of the GOP pack, was clearly trying to seize the narrative -- but Rubio quickly and effectively counterpunched, owning the moment.
Rubio 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 they 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.
Tonight's 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.
Carson, who has made several controversial comments about homosexuality and Muslims, said he has no plans to 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."
The other big target tonight: the media.
And as he did through much of the debate, Rubio led the attack, joined by Trump and Ted Cruz. In one memorable line, Rubio said Democrats don't need a super PAC because they enjoy support from the mainstream media.
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."
-- By MJ Lee; CNN's Jeff Zeleny contributed to this report.