Donald Trump and Ben Carson defend their White House bids in TV debate
Outsiders Donald Trump and Ben Carson defended the seriousness of their White House bids in the third Republican presidential debate.
Meanwhile Jeb Bush targeted Marco Rubio in a fight for control of the Republican Party's establishment wing at the event in Boulder, Colorado.
Mr Trump, a brash billionaire businessman, dominated the polls for months.
But Mr Carson has recently taken the momentum and overtook the tycoon in polls in Iowa, where the first of the state-by-state contests will be held.
Neither Mr Trump nor Mr Carson is a career politician but both have capitalised on anti-Washington, anti-establishment feeling among many conservatives.
Mr Trump largely refrained from personal attacks on his rivals, which has been a signature of his campaign, even taking a light touch with Mr Carson.
He bristled when asked by a debate moderator if his policy proposals, including building a wall along the US-Mexico border and deporting everyone who is in the US illegally, amounted to a "comic book" campaign.
"It's not a very nicely asked question, the way you ask it," Mr Trump responded. Then he defended his proposals as reasonable.
Mr Carson, a soft-spoken retired neurosurgeon, stuck to his low-key style and 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.
He had earlier said that someone making 10 billion dollars would pay 1 billion dollars in taxes. During the debate he floated the idea of a 15% flat rate.
Critics have questioned whether the government could still raise enough revenue under that type of flat tax system to pay for federal programmes.
Mr Bush and Mr Rubio clashed for control of the party's more mainstream wing in the debate, which comes some three months before the first votes are cast in the race, leaving much time for things to change.
Mr Bush, the former Florida governor and son and brother of former presidents, was once seen as the main Republican contender.
But he entered the debate in the middle of the most difficult stretch of his White House campaign.
By attacking Mr Rubio, a senator, for his patchy voting record on Capitol Hill, Mr Bush signalled that he sees his fellow Floridian as the candidate most likely to block his political path.
"Marco, when you signed up for this, this was a six-year term and you should be showing up for work," said Mr Bush, who is struggling to right his campaign after being forced to slash spending in response to slower fundraising.
Mr Rubio, who has had a close relationship with Mr Bush, responded sharply: "Someone has convinced you that attacking me is going to help you."
The Republican nominee will most likely face Hillary Clinton, former first lady and secretary of state.
The overwhelming favourite for the Democratic nomination, her campaign received a boost from a strong debate performance two weeks ago.
Ohio governor John Kasich and New Jersey governor Chris Christie are each seeking to break through with more mainstream Republican voters.
Also on stage were former Arkansas governor Mike Huckabee, Kentucky senator Rand Paul and former technology executive Carly Fiorina, the star of the second Republican debate.
Ms Fiorina, the former Hewlett Packard chief executive, has struggled to capitalise on that strong performance and has faded towards the back of the pack.