BOULDER, Colorado (BLOOMBERG/AFP) - Republican presidential hopefuls traded bald attacks on Wednesday (Oct 28) in their third primary debate, with front runners Donald Trump and Ben Carson facing intense criticism for peddling "fantasy" economic policies.

The party's third primary debate, less than 100 days before the first statewide nominating contests, also saw Mr Jeb Bush breathe fire at a former ally, telling Senator Marco Rubio he should resign from the Senate if he keeps skipping votes while running for president.

The 10 White House candidates on stage were quick with the personal barbs, a sign of the rising tension between the country's established politicians and political neophytes who currently top the polls in the race for 2016.

The Republican outsider front runners were slammed at the outset of the debate as the moderators and some of their opponents pushed for details of their tax and budget proposals and raised questions about their inexperience in government.

Ohio Governor John Kasich, who before the debate had hinted that he would be more aggressive, warned that his party was on the verge of "picking someone who perhaps cannot do this job", and after Mr Carson defended his tax rate, exploded. "This is the fantasy that I talked about at the beginning," he said.

"Folks, we got to wake up," he added, shaking his head. "We cannot elect somebody that doesn't know how to do the job. You have to pick somebody who has experience."

The language appeared clearly aimed at Mr Trump, a real estate billionaire, and the retired neurosurgeon Carson. Mr Kasich said both Mr Trump's and Mr Carson's tax plans were "fantasy tax games" that would cost the country trillions of dollars.

Even the debate moderator joined in, asking Mr Trump at the outset if he was running a "comic book version" of a campaign - notably by promising to keep out immigrants with a wall on the Mexican border.

Mr Trump's pushback was tame compared to his retorts in earlier debates. "You can have him," he sneered of Mr Kasich.

Mr Kasich also attacked Mr Trump for his proposal to deport an estimated 11 million illegal immigrants. Mr Trump immediately shot back at Mr Kasich, zeroing in on his tenure as an investment banker in between his terms as a member of Congress and Ohio governor.

"He was a managing general partner with Lehman Brothers when it went down the tubes," Mr Trump said.

The debate took place at a volatile time in the race, as Mr Trump's grip appeared to be loosening on the front-runner status he has enjoyed until now. The session was the first since several polls showed the lead in the Republican race may be changing, with Mr Carson moving up.

The retired doctor led in several recent polls of Iowa Republicans, including one by Bloomberg Politics and the Des Moines Register.

This week he dislodged Mr Trump from the top spot in a national poll. A New York Times/CBS News survey released Tuesday showed Mr Carson as the choice of 26 per cent of Republican primary voters, compared to 22 per cent for Mr Trump, a difference within the margin of error.

Another intriguing subplot quickly emerged as two Florida Republicans in the race, US Senator Marco Rubio and former governor Jeb Bush, traded jabs over Mr Rubio missing votes in Washington. Both came into the debate under pressure to perform: Mr Bush stood next to Mr Trump during the first two debates, but has displaced after a drop in the polls by his Miami neighbour.

Mr Rubio, whose preparations included weight-lifting in a nearby hotel gym about 24 hours before the debate and the release of a playful fantasy football-themed video, has won good marks for his previous debate performances but has struggled to translate that into a jump in the polls.

Mr Bush struck first, echoing an editorial this week from the Fort Lauderdale Sun-Sentinel that called on Mr Rubio to resign from the Senate for missing votes, pointing out that the Florida newspaper endorsed him five years earlier.

"You ought to show up for work," Mr Bush told his former political protege, noting that he is one of Mr Rubio's constituents.

"The Senate, what is it, like a French work week?" Mr Bush said, referring to the chamber's habit of working Tuesday to Thursday.

"You get like three days where you have to show up?" Mr Bush said. "You can campaign, or just resign and let someone else take the job," he lectured the lawmaker who served as speaker of the Florida state House when Mr Bush was governor.

"There are a lot of people living paycheck to paycheck in Florida as well. They're looking for a senator that will fight for them each and every day."

But Mr Rubio got the better of the exchange, judging by the reaction from the conservative audience. He earned applause for blaming the newspaper editorial on a "double standard" for Republicans in the media, again when he dismissed Mr Bush's attack as a one of political expediency, and a third time after criticising Democratic front runner Hillary Clinton.

"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 to Mr Bush. "My campaign is going to be about the future of America, it's not going to be about attacking anyone else on the stage."

The debate, hosted by CNBC and focused on the economy, seemed to be a golden opportunity for Mr Trump, a billionaire businessman. Polls show voters trust him as much-or more-as other Republican presidential candidates on economic issues.

Mr Trump defended his business use of the nation's bankruptcy laws in his businesses, specifically in Atlantic City, New Jersey, as he noted that he has never personally filed for bankruptcy protection. "I used the laws of the country to my benefit, I'm sorry," he said.

He isn't alone among those on the debate stage with business experience. Among the nine other contenders in the prime-time debate are Ms Carly Fiorina, the former Hewlett-Packard chief executive officer who regularly touts her business experience, and Mr Bush, who has built his campaign around made a promise of 4 per cent economic growth.

He's spent much of the decade since he left office in the private sector, consulting for Lehman Brothers and Barclays. Mr Bush has also served on the boards of companies including Tenet Healthcare, Rayonier, Empower Software and Geo Fossil Fuels.

The debate was held hours after Federal Reserve policymakers indicated that they would consider raising the benchmark interest rate at their next meeting in December, as the economy has recently logged a slower pace of job gains, and hours after the US House of Representatives approved a budget deal negotiated by President Barack Obama and leaders of the Republican party but opposed by many conservative Republicans.

Senator Ted Cruz received perhaps the most rousing applause of the night when he sidestepped a question about his opposition to the budget deal reached by Congress and President Obama, and instead launched into a critique of the media.

"You know, let me say something at the outset. The questions that have been asked so far in this debate illustrate why the American people don't trust the media," Mr Cruz said. "This is not a cage match, and if 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-how about talking about the substantive issues people care about."

Held in a state that swings back and forth between Republicans and Democrats - electing a Republican senator two years after handing its electoral college votes to Mr Obama - the debate's venue in many respects represents the political cross-currents in the country.

It is being held in the Coors Events Centre, named for a family that has financed conservative causes and candidates, on the campus of the University of Colorado in a town that locals affectionately dub the "People's Republic of Boulder".

Located about 55km north-west of Denver, Boulder was a destination for participants in 1960s counter-culture. It was among the first communities in the nation to issue same sex marriage licences in the 1970s, and is known to be the site of clothing-optional bike rides.

One of the town's unofficial mottos is, "Keep Boulder Weird".

Colorado is one of the first states to legalise recreational marijuana use, and it's a state where one in five residents is Hispanic, a voting bloc that Mr Bush and Sen Rubio are especially trying to attract to the party.

It's also a state that has seen two of the most horrific gun massacres in recent history, and where two state lawmakers were recalled for supporting stricter gun control laws.

Other candidates on stage were former Arkansas governor Mike Huckabee, New Jersey Governor Chris Christie and Senator Rand Paul of Kentucky.

Earlier in the evening, the undercard debate featured four presidential candidates lagging in the polls: Louisiana Governor Bobby Jindal, Senator Lindsey Graham of South Carolina, former New York governor George Pataki and former Senator Rick Santorum of Pennsylvania. All four have been stuck in the so-called happy hour debate for all the previous debates with no sign of an uptick on their polling.

The candidates in the earlier debate attacked both Democrats and Washington. "We need an outsider," said Mr Pataki, who served as a mayor, assemblyman, state senator and New York governor. Mr Graham wisecracked at the expense of his fellow senator Bernie Sanders, a socialist who is running behind Hillary Clinton in the race for the Democratic nomination.

"The No. 2 guy went to the Soviet Union on his honeymoon and I don't think he ever came back," Mr Graham said.

One or more of those on the main stage tonight may not make the prime-time show at the next gathering on Nov 10 in Milwaukee. For that session, candidates will have to earn 2.5 per cent or higher support in the four newest national polls prior to the debate.

The first two Republican debates attracted record-breaking audiences for the cable networks that aired them, a landmark for which Mr Trump has taken credit.

But even the one-time reality TV star may have a hard time drawing eyeballs to this debate: It's airing at the same time as game 2 of Major League Baseball's World Series match-up between the Kansas City Royals and the New York Mets.