US Republicans spar in third fiery presidential debate
Marco Rubio has been offering measured answers while Donald Trump and Ben Carson defend their proposed policies.
US Republicans have traded blows in a heated presidential debate in Colorado that featured several angry exchanges.
Frontrunners Donald Trump and Ben Carson, with no political experience, were under attack from the start.
Ohio Governor John Kasich condemned their "fantasy tax plans" and added: "We can't elect someone who doesn't know how to do the job,"
Mr Carson, a retired neurosurgeon who has edged past Mr Trump in national polls, had a quiet night in Boulder.
His tax plan, which is based on biblical tithes, was decried as "fantasy" by Mr Kasich, who also condemned Mr Trump's plan to deport 11 million undocumented immigrants and build a wall on the Mexico border.
Political friendships were strained by some of the testy exchanges, notably between former Florida Governor Jeb Bush and Florida Senator Marco Rubio.
Mr Bush urged Mr Rubio, once his protege, to resign from the senate because of his poor voting record.
Texas Senator Ted Cruz got the biggest applause of the night when he attacked the hosts, CNBC, because he thought the questions were confrontational.
Analysis - Nick Bryant, BBC News, Boulder
Twin contests played out on the stage in Colorado - a fight to become the candidate of the Republican establishment and a battle to become the standard bearer of the radical right.
In the first contest, Jeb Bush delivered another listless performance that will deeply worry his donors, and a pre-meditated decision to attack his friend and rival Marco Rubio for absenteeism from his day job, as the Florida Senator, backfired badly. It seemed so contrived, as Rubio, a big winner tonight, deftly pointed out.
In the establishment contest, it was the defining exchange of the night, and will enhance Rubio's growing stature and further diminish Bush. The New Jersey Governor Chris Christie also outstripped Bush.
As for a rumble in the Rockies between Donald Trump and the candidate who dislodged him in the polls, Ben Carson, it never unfolded. Trump was low-key - it seemed almost that he is tiring of the process. Carson disappeared for much of the debate.
Perhaps sensing a chance to impose himself, the Texas Senator Ted Cruz delivered an impassioned attack on the moderators of the debate and the media more broadly for its liberal bias. Speaking of his born-again father will have impressed evangelicals. This was the best two hours of the Cruz campaign so far.
But the main headline of the evening comes from Jeb Bush. He needed to energise his troubled campaign tonight, and he failed abysmally.
Other highlights included:
- Mr Trump denied his economic plan was a "comic-book version" of a presidency campaign
- Mr Rubio touted his humble upbringings and said he would fight for struggling Americans
- New Jersey Governor Chris Christie said the government "stole" Social Security taxes
- Mr Trump called gun-free zones "target practice" for the mentally ill
- he denied calling Mr Rubio the "personal senator" of Facebook founder Mark Zuckerberg, although the description is on his own website
- Carly Fiorina accused Democratic candidate Hillary Clinton of hypocrisy because every single policy of hers is "bad for women"
The four lowest-polling Republican candidates squared off in an early debate.
Mr Graham, the South Carolina senator, got the most laughs, especially when he said Democratic candidate Bernie Sanders "went to the Soviet Union on his honeymoon and I don't think he ever came back".
Primary voting begins in February in Iowa, 10 months before the nation goes to the polls to vote for its new president.
