(AP) -- The Republican candidates for president will gather Wednesday for their third debate amid fresh volatility in an already chaotic race, with Ben Carson surging past Donald Trump in Iowa and one-time front-runner Jeb Bush under pressure to prove he's still a viable candidate for the GOP nomination.

The soft-spoken Carson has been a low-key presence in the first two GOP debates, but the retired neurosurgeon is likely to get more attention from moderators -- as well as his fellow candidates -- after a series of preference polls show him atop the field in Iowa.

Trump has already shown he's eager to take on Carson, jabbing him for his speaking style and raising questions about his Seventh Day Adventist faith.

"We'll see how Ben holds up to the scrutiny," Trump said Tuesday on MSNBC.

Meanwhile, Bush will be grasping for momentum after one of the most trying stretches of his White House campaign. Slower-than-expected fundraising has led Bush to slash spending and overhaul his campaign structure, and he's voiced frustration with the way the unusual race has progressed.

If the election is going to be about fighting to get nothing done, he says, "I don't want any part of it."

There will be 10 candidates on stage in the prime-time debate in Boulder, Colorado, all seeking a share of a smaller spotlight: this debate on CNBC will run for only two hours after the last affair went on for more than three.

Among the participants are two senators -- Florida's Marco Rubio and Ted Cruz of Texas. Rubio has sought to capitalize on Bush's stumbles, but faces his own financial concerns. Cruz is positioning himself to inherit Trump's supporters if the real estate mogul's campaign collapses.

Taken together, it's a Republican field that remains crowded and unwieldy three months before the lead-off Iowa caucuses and New Hampshire primary. The political rookies appealing to voter anger with Washington have ceded no ground and establishment politicians are still waiting for the race to turn their way -- and increasingly wondering if it ever will.

Trump remains the dominant force, commanding media attention, drawing large crowds and leading in most early voting states. But his dip in Iowa has prompted some speculation among Republicans that the tide could be turning against the bombastic billionaire.

"His only hope of staying competitive is to entertain voters with his provocateur-in-chief routine right up until Election Day," said Josh Holmes, a former adviser to Senate Majority Leader Mitch McConnell. "He's the one candidate where 'acting presidential' actually has a detrimental effect on his campaign."

While Carson is unknown to many Americans, he's built a loyal following with tea party-aligned voters and religious conservatives. His campaign has started running new television advertisements in early voting states that center on his experience as a doctor and highlight his status as a political outsider.

Carson has raised eyebrows with his incendiary comments about Muslims and references to Nazis and slavery on the campaign trail, rhetoric he's made no apologies for. His standing in early states has only appeared to strengthen with each controversial comment.

Carson's biggest weakness may be his glaring lack of specific policy proposals. The issues listed on his campaign website are vague, including a tax plan that calls for a "fairer, simpler, and more equitable" system. On foreign policy, he's said, "all options should remain on the table when dealing with international bullies," such as Russian President Vladimir Putin.

Carson could be pushed Wednesday on domestic policy, with debate host CNBC promising to focus on economic issues, including taxes and job growth.

Policy discussions are usually a welcome refuge for Bush, the wonky former Florida governor. But his challenge Wednesday is less about highlighting his mastery of the issues and more about showing his supporters he has the temperament to fight through a long and grueling primary.

"You've got a guy here speaking from experience, speaking with knowledge about issues, speaking with a reasonable approach to matters," said Pat Hickey, a Bush supporter from Nevada. "The problem, though, is: do those things seem to matter to the electorate?"

With a well-funded super PAC standing by, Bush doesn't appear to be on the brink of a campaign collapse, even if he performs poorly in the debate. But a stronger performance could help soothe supporter anxiety.

Also on stage event Wednesday will be Ohio Gov. John Kasich, New Jersey Gov. Chris Christie, former Arkansas Gov. Mike Huckabee, Kentucky Sen. Rand Paul and former technology executive Carly Fiorina. Each will be eager for the kind of standout moment that Fiorina had in the second debate to jumpstart their campaigns.

The four lowest-polling candidates will participate in an earlier undercard event: South Carolina Sen. Lindsay Graham, former Pennsylvania Sen. Rick Santorum, Louisiana Gov. Bobby Jindal and former New York Gov. George Pataki. None has gotten close to breaking into the upper tier of candidates.

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Follow Julie Pace on Twitter at: http://twitter.com/jpaceDC