During a Tuesday rally in Iowa, Donald Trump all but dropped to his knees seeking more support.
Donald Trump, the longtime frontrunner in the Republican presidential race, geared up for battle Wednesday in a third nationally televised candidates' debate, with outsider Ben Carson hot on his heels.
Millions of Americans were set to tune in to watch 10 White House hopefuls spar in an arena at the University of Colorado in Boulder, less than 100 days before the first statewide nominating contests.
As traditional candidates struggle to make headway against strong populist currents in their party, Carson appears to be catching up to Trump.
The retired neurosurgeon has overtaken Trump in the state of Iowa, and edged ahead of him in one national poll released Tuesday.
This marked the latest sign of slippage for the billionaire tycoon, and a portent of fireworks Wednesday as the hyper-competitive Trump seeks to reaffirm his position atop the Republican pyramid.
During a Tuesday rally in Iowa, Trump all but dropped to his knees seeking more support.
"Iowa, will you get your numbers up please?" he said. "I promise you I will do such a good job."
With his star rising, Carson will likely come under pressure to expand on his political platforms, and explain his controversial comments about Hitler, gays and gun violence.
And Trump in particular could come out swinging against Carson, whom he has criticized as low-energy.
The real estate mogul joked recently that his main rival did not realize he had surged in polls because he was too busy "sleeping."
Carson said he would not be dragged into the "mud pit." On Tuesday a new Carson ad emerged showing the soft-spoken doctor and author -- who is also the only African-American in the race -- in a buoyant mood, relishing his outsider status.
"I'm running for president, and I'm very much outside the box," he beamed.
Struggling to break out
Ten candidates will take the stage for the main event: Trump, Carson and former Florida governor Jeb Bush; Senators Marco Rubio, Ted Cruz and Rand Paul; former Hewlett-Packard chief executive Carly Fiorina, who performed well at the previous debate; Arkansas ex-governor Mike Huckabee; and Governors John Kasich of Ohio and Chris Christie of New Jersey.
Cable network CNBC, which is broadcasting the prime time event starting at 0000 GMT, says its debate will focus on economic issues, including tax policy, federal spending and job growth.
Four candidates, all polling at one percent or less, kicked off the evening with an undercard debate, sparring over taxes, job and entitlements and highlighting core conservative principles.
Signalling he might be out of step with today's Republicans, Senator Lindsey Graham repeated his openness to immigration reform as a weapon against the Democrats in 2016.
Louisiana Governor Bobby Jindal said he supported companies providing paid family leave and raising wages, "but the government can't wave a magic wand and make that happen."
One open question for the evening was whether Bush, the son and brother of two presidents, would go after Trump.
Despite raising mountains of money as the early frontrunner, Bush has so far failed to break out of the pack, and an argument could be made that he would benefit from showing some backbone and standing 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."
The main event is also 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 stands third in the RealClearPolitics average of opinion polls, at nine percent, well behind Trump (26.8) and Carson (22.0) but ahead of Bush (7.0) and Cruz (6.6).