BOULDER, Colo. (AP) - Jeb Bush and Marco Rubio fought for control of the Republican's establishment wing in Wednesday night's third GOP debate, as insurgent outsiders Donald Trump and Ben Carson defended the seriousness of their White House bids, underscoring the volatile two-track fight for the party's presidential nomination.
But in an economic policy-focused debate, Trump and Carson at times faded to the background during the two-hour contest.
Bush, once seen as the top Republican contender, entered the debate desperately in need of a strong performance to right his sluggish campaign and soothe his supporters' anxiety. He quickly targeted Rubio for his spotty voting record on Capitol Hill, signaling that he sees the Florida senator as the candidate most likely to block his political path.
"Marco, when you signed up for this, this was a 6-year term and you should be showing up for work," said Bush, who was forced to slash campaign spending in response to slower fundraising. "You can campaign, or just resign and let someone else take the job."
Rubio, who has had a close relationship with Bush, responded sharply: "The only reason you're doing it is that we're running for the same position and someone has convinced you that attacking me will help you."
Three months before primary voting begins, the Republican contest remains crowded and unwieldly. Yet the contours of the race have been clarified, with outsiders capitalizing on voter frustration with Washington and candidates with political experience hoping the race ultimately turns their way.
Trump, the brash real estate mogul, has dominated the Republican race for months, but was a less of a factor Wednesday night than in the previous two debates. He largely refrained from personal attacks on his rivals, which has been a signature of his campaign, even taking a light touch with Carson, who has overtaken him in recent Iowa polls.
Carson, the soft-spoken retired neurosurgeon who came into the debate with a burst of momentum, stuck to his low-key style. He sought to explain his vague tax policy, which he has compared to tithing, in which families donate the same portion of their income to their church regardless of how much they make. And he insisted he had no involvement with supplement maker Mannatech, although he acknowledged using its product and giving paid speeches for the company, which has faced a legal challenge over health claims for its products.
Carson said it was absurd to allege he's connected to the company. "If someone put me on their home page, they did it without permission," he said.
Trump bristled when asked by a debate moderator if his policy proposals, including building a wall along the U.S.-Mexico border and deporting everyone who is in the U.S. illegally, amounted to a "comic book" campaign. And he defended his record in the private sector despite having to declare bankruptcy, casting it as a business technique.
"I've used that to my advantage as a businessman," Trump said. "I used the laws of the country to my benefit."
Texas Sen. Ted Cruz has been circling Trump for months, seeking to position himself as the heir to the businessman's supporters if he fades. While Cruz holds office in the U.S. Senate, he's cast himself as anti-establishment and a thorn in the side of GOP leaders.
Cruz garnered enthusiastic applause when he criticized debate moderators for trying to stir up fights among the candidates, casting it as a sign of media bias against Republicans -- a popular line with GOP voters.
The jumbled GOP field is a stark contrast to the Democratic contest, where Hillary Rodham Clinton is strengthening her front-runner status over Vermont Sen. Bernie Sanders. Campaigning in New Hampshire ahead of the GOP debate, Clinton said the Republican contests are like a "reality TV show but the cast of characters are out of touch with actual reality."
Wednesday's debate in Colorado, an important general election state, focused on economic policy, including taxes and job growth.
Rubio turned questions about his personal financial struggles, including recently liquidating his retirement account, into an opportunity to tout his compelling personal story. The son of Cuban immigrants, Rubio said he didn't inherit money from his family and knows what it's like to struggle to pay loans and afford to raise a family.
"I know what it's like to owe that money," Rubio said. "I'm not worried about my finances. I'm worried about the finances of everyday Americans."
The feud between Bush and Rubio has been simmering for months, driven by the former Florida governor's concern that his talented protege could eclipse him. Rubio is among the Republican field's most talented politicians, and at age 44 he could represent the future in a way the son and brother of presidents cannot.
Bush has cast himself as a policy wonk and delivered measured answers to questions on tax policy and the nation's budget. But other than his sharp critique of Rubio, Bush had few of the standout moments his supporters were seeking.
Ohio Gov. John Kasich and New Jersey Gov. Chris Christie have each sought to break through with more mainstream voters. Kasich in particular was aggressive from the start in bemoaning the unexpected strength of unorthodox candidates.
"We are on the verge of perhaps picking someone who cannot do this job," Kasich said.
Christie, whose campaign has so far failed to meet expectations, cast himself as best qualified to defeat Clinton in the general election.
"You put me on the stage with her next September and she won't get within 10 miles of the White House," he said.
Also on stage were former Arkansas Gov. Mike Huckabee, Kentucky Sen. Rand Paul and former technology executive Carly Fiorina, the star of the second GOP debate. Fiorina, the former Hewlett Packard CEO, has struggled to capitalize on that strong performance and has faded toward the back of the pack.
The four lowest-polling candidates participated in an earlier undercard event: South Carolina Sen. Lindsay Graham, former Pennsylvania Sen. Rick Santorum, Louisiana Gov. Bobby Jindal and former New York Gov. George Pataki. None has gotten close to breaking into the upper tier of candidates.
AP writers Josh Lederman and Lisa Lerer contributed to this report.