BOULDER, United States (AFP) - White House hopeful Jeb Bush mustered the ultimate American put-down as he clashed with a former ally at the Republican debate on Wednesday (Oct 28): He accused him of putting in a French work week.

Millions of Americans were tuned 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.

Florida Senator Marco Rubio - who like Mr Bush hopes to steal a march on current front runners Donald Trump and Ben Carson - has come under fire over perceptions he neglected his duties as he focuses on 2016.

"Literally, the Senate, is it a French work week?" Mr Bush hit out in a fiery exchange with his Florida neighbour early in the debate in Boulder, Colorado. "Just resign and let somebody else take the job. There are a lot of people who are living paycheck to paycheck in Florida."

Many consider Mr Rubio to be the most serious establishment challenger beyond Mr Bush to go up against Mr Carson or Mr Trump, neither of whom have held elected office.

Mr Rubio responded to Mr Bush by saying many White House hopefuls - including Senator John McCain - had missed Senate votes as they focused on the presidential race.

"Jeb, let me tell you, I don't remember you ever complaining about John McCain's vote record," said Mr Rubio. "Someone convinced you attacking me is going to help you."

"I will continue to have tremendous admiration for Governor Bush. I'm not running against anyone on the stage. I'm running for president."

Mr Rubio stands third in the RealClearPolitics average of opinion polls, at 9 per cent, well behind Mr Trump (26.8) and Mr Carson (22.0) but ahead of Mr Bush (7.0).

But all eyes will be on Mr Trump, the longtime front runner in the Republican presidential race, who geared up for battle, with outsider Mr Carson hot on his heels.

As traditional candidates struggle to make headway against strong populist currents in their party, Mr Carson appears to be catching up to Mr Trump.

The retired neurosurgeon has overtaken Mr Trump in the state of Iowa, and edged ahead of him in one national poll released on Tuesday.

This marked the latest sign of slippage for the billionaire tycoon, and a portent of fireworks on Wednesday as the hyper-competitive Mr Trump seeks to reaffirm his position atop the Republican pyramid.

During a Tuesday rally in Iowa, Mr 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, Mr Carson will likely come under pressure to expand on his political platforms, and explain his controversial comments about Hitler, gay people and gun violence.

And Mr Trump in particular could come out swinging against Mr Carson, whom he has criticised as low-energy.

The real estate mogul joked recently that his main rival did not realise he had surged in polls because he was too busy "sleeping".

Mr 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.

Ten candidates will take the stage for the main event: Mr Trump, Mr 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, says its debate will focus on economic issues, including tax policy, federal spending and job growth.

Four candidates, all polling at one per cent 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 Mr Bush, the son and brother of two presidents, would go after Mr Trump.

Despite raising mountains of money as the early front runner, Mr 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 Mr Trump.

But one major donor said Mr 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 Mr Bush. "He doesn't want to be in this... cesspool of taunts and nonsense."