WASHINGTON (AFP) - Republican White House frontrunner Donald Trump and nine other 2016 hopefuls do battle on Wednesday (Oct 27) in their third nationally televised primary debate, as rivals seek a political opening against the caustic billionaire.
Millions of Americans are expected to watch the debate, which will take place at an arena at the University of Colorado in Boulder, as the field moves toward the first statewide votes in the primary process next February.
As establishment-leaning candidates struggle to make headway against strong populist currents in their party, outsider Ben Carson has been nipping at Trump's heels in the polls.
Carson, a retired neurosurgeon and the only African-American in the race, edged ahead of Trump in a new national New York Times/CBS News survey released on Tuesday.
It marked the latest sign of slippage for the real estate tycoon, and a portent of potential fireworks on Wednesday as the hyper-competitive Trump will seek to restore the old order with him securely atop the Republican pyramid.
He could come out swinging against his rivals, particularly Carson whom he has criticised in recent days as a low-energy candidate.
Trump joked at a rally that Carson did not realise he had surged in the polls because he was too busy "sleeping."
And he retweeted a post saying that "Gentle Ben is no match for (Russian President Vladimir) Putin or if the truth be told even for Hillary" Clinton, the presumptive Democratic nominee.
Don't expect Carson to plunge into the muck. He said on Sunday he would not be dragged into the "mud pit," and his campaign has remained above the fray, highlighting the candidate's serious, measured temperament.
And a new Gallup poll shows he has higher favorability ratings than any Republican running.
STRUGGLING TO BREAK OUT
One open question for the evening debate - which begins at 0000 GMT Thursday (8.00am Singapore time), and is to be broadcast by cable network CNBC - is whether Jeb Bush, the son and brother of two presidents, will take Trump's bait.
Despite raising mountains of money and starting off as the frontrunner, Bush has so far failed to set himself apart from the pack, and the argument could be made that he would be well served to show some backbone and stand up to Trump.
But one major donor said Bush is taking the long view, expecting the Trump "phenomenon" to fade once voters start paying closer attention to policy specifics.
"He wants to govern," the donor said of Bush. "He doesn't want to be in this... cesspool of taunts and nonsense."
CNBC says its debate, titled "Your Money, Your Vote," will focus on economic issues including tax policy, federal spending and job growth.
Ten candidates will take the stage for the prime time event: Trump, Carson and Bush; Senators Marco Rubio, Ted Cruz and Rand Paul; former Hewlett-Packard chief executive Carly Fiorina; Arkansas ex-governor Mike Huckabee; and Governors John Kasich of Ohio and Chris Christie of New Jersey.
Four other low-polling candidates will participate in a undercard debate from 2200 GMT Wednesday.
The main event is seen as a prime opportunity for Rubio, whom many consider the most serious establishment challenger beyond Bush to go up against Trump and Carson, neither of whom have held elective office.
Rubio is young, well versed in foreign policy, and the son of Cuban immigrants who speaks fluent Spanish, which could serve him well in attracting Latino votes.
He stands third in the RealClearPolitics average of opinion polls, at nine per cent, well behind Trump (26.8) and Carson (22) but ahead of Bush (7.0), Cruz (6.6) and Fiorina (5.8).