text: 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 proposal, which is based on biblical tithes, was decried by Mr Kasich, who also dismissed Mr Trump's plan to deport 11 million undocumented immigrants and build a wall on the Mexico border.
  The five key confrontations
  Political friendships were strained by some of the testy exchanges, notably one 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.
  The media were also in the firing line - Texas Senator Ted Cruz got the night's biggest applause when he attacked the hosts, CNBC for stirring confrontation.
  "The questions that have been asked so far in this debate illustrate why the American people don't trust the media. This is not a cage match."
  The hostility against CNBC continued after the debate when Republican National Committee chairman Reince Priebus complained about the "gotcha questions".
  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 premeditated 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:
  The four lowest-polling Republican candidates squared off in an early debate.
  Lindsey Graham, a senator from South Carolina, 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.
target: US Republicans have traded blows in a heated presidential debate in Colorado that featured several angry exchanges.