Republican White House frontrunner Mitt Romney stumbled a day after his Florida triumph with a gaffe about poor Americans, while rival Newt Gingrich eyed a boost Thursday from Donald Trump.

Populist real estate tycoon and reality TV star Trump, who flirted with running for the presidency himself last year, was expected to throw his weight behind Gingrich at a "major announcement" in Las Vegas.

Both Republican frontrunner candidates headed from Florida to the western battleground state of Nevada a day after Romney soundly defeated Gingrich in the Sunshine State, retaking the lead in the White House nomination race.

But the multi-millionaire former Massachusetts governor triggered immediate criticism for a controversial comment about America's poorest citizens, which threatened to slow his momentum.

"I'm not concerned about the very poor. We have a safety net there. If it needs repair, I'll fix it," said Romney, a wealthy businessman who last week acknowledged earning $20 million in 2010.

"I'm not concerned about the very rich, they're doing just fine," the former venture capital boss told CNN. "I'm concerned about the very heart of America, the 90-95 percent of Americans who right now are struggling."

The remarks came as the race for the right to take on Democratic President Barack Obama in November widened into more states, and were quickly seized upon by his opponents.

Gingrich, who lost to Romney by more than 14 percentage points in Florida, fired back at his rival.

"I am fed up with politicians in either party dividing Americans against each other. I am running to be president of all the American people and I am concerned about all of the American people," Gingrich said in Nevada.

Romney swiftly tried to explain his remarks, telling reporters traveling with him on his plane that they should consider everything he said, rather than just part of the sentence.

"Of course I'm concerned about all Americans... poor, wealthy, middle class, but the focus of my effort will be on middle income families who I think have been most hurt by the Obama economy," he said later.

At a campaign stop in Las Vegas on Wednesday evening Romney was back on track, drawing cheers with his usual stump speech and taking digs at Obama's policies, including his backing for "European-style" health care.

Romney also attacked the "naivety" of an announcement Wednesday by Defense Secretary Leon Panetta that the US plans to end its combat mission in Afghanistan in 2013, one year before most US troops are due to withdraw.

"Why in the world do you go to the people that you're fighting with, and tell them the day you're pulling out your troops? It makes absolutely no sense," he told hundreds of supporters in a suburban Vegas warehouse.

On Thursday all eyes will be on Trump, who scheduled an announcement for 12:30 pm (2030 GMT) -- which the New York Times and other media said would be an endorsement for Gingrich.

Public backing from Trump, host of the popular celebrity version of reality TV show "The Apprentice," could give the former House speaker a higher media profile and aid him in fundraising.

Nevada holds its caucus vote on Saturday, along with a similar caucus in Maine, while five other US states will hold votes in the coming month.

As the first western state in the 2012 Republican nomination battle, Nevada thrusts new demographics and issues into the limelight, including concerns over illegal immigration, the environment and natural resources.

Like Florida, the Silver State has been badly hit by the recession, with soaring unemployment and a housing market devastated by the 2008 meltdown.

Romney, who won handily here in the 2008 White House race, can expect a boost from fellow Mormons. Four years ago, 26 percent of Nevada Republican caucus voters were Mormon and 95 percent of them backed Romney.

The two other Republicans still in the race -- Texas representative Ron Paul and Pennsylvania's former senator Rick Santorum -- gave up on Florida and had already been busy canvassing in Nevada before Romney and Gingrich arrived.