Updated 5:41 p.m. ET

Donald Trump today formally endorsed Mitt Romney for president, saying the former Massachusetts governor is "smart" and "tough."

At a news conference at Trump's hotel in Las Vegas, the real estate developer and reality TV star appeared with Romney and his wife, Ann. The endorsement comes two days before the Nevada caucuses, where a new statewide poll shows Romney with a huge lead over Newt Gingrich.

Romney said he was honored to get Trump's endorsement and saluted the developer's work in the private sector and his stance on China.

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"Donald Trump's endorsement of me today sends a strong signal that he believes we have the best chance to make President Obama a one-term president," Romney said. "We share both a background in the private sector and a belief that Washington needs to get out of the way of small business."

Trump's formal endorsement of Romney ended a wild ride of speculation about the real estate mogul's intentions.

Earlier, the Associated Press and other news organizations reported Trump was set to back Newt Gingrich, citing sources close to the Gingrich campaign.

Does Trump's endorsement matter? A Pew Research Center poll last month showed 64% of Republican and GOP-leaning voters said his backing would make no difference in how they vote in the presidential election.

Ron Paul's campaign issued a statement asking "why anyone would care" about Trump. "Please explain to Republican voters in Nevada why they should consider the opinion of a billionaire from New York who endorsed the archenemy of all Republicans in Nevada, and really the enemy of all Republicans" in the United States, the statement said, in an apparent reference to Senate Majority Leader Harry Reid, D-Nev.

But the seesawing about Trump's endorsement is another example of how the developer/reality TV star has been able to inject himself in the political debate. He flirted with the idea of running for the GOP nomination last year, and made President Obama's birth certificate a central focus.

Trump also made headlines when it was announced that he would moderate a debate before the Iowa caucuses, organized by the conservative website NewsMax. But several candidates, led by Paul and Jon Huntsman, balked at participating in what they believed would become a political circus.

In an interview with CNN after the endorsement, Trump played down previous criticisms he made about the GOP candidate, saying that was "political talk." CNN showed one clip of Trump denouncing Romney as a "small-business guy." And in an interview with The Daily Beast, Trump said that Romney was "going to lose" the presidency because "he doesn't resonate."

Trump struck a different tone in his CNN interview today. "He has done a good job," Trump said about Romney. "He's a warm, smart, tough cookie. We need someone tough."

Trump told CNN's Wolf Blitzer that he would do "whatever necessary" to help Romney, including making political donations or giving speeches on his behalf.