WASHINGTON (Reuters) - Businessman and reality TV personality Donald Trump is endorsing Mitt Romney in the race for the 2012 Republican nomination, CNN reported on Thursday, a day after U.S. media reported the real estate mogul would be endorsing Romney's rival Newt Gingrich.

Trump, himself an on-again/off-again candidate for president and former Republican, will announce his support for Romney on Thursday in Las Vegas, the cable television network reported, citing sources with knowledge of the endorsement.

The CNN report was a direct contradiction to reports on Wednesday, citing sources close to Gingrich's campaign, that Trump would endorse the former speaker of the U.S. House of Representatives in Las Vegas on Thursday.

Michael Cohen, Trump's political adviser, said he was not at liberty to reveal what Trump would say at his news conference. "It's only a guessing game because the press are making it a guessing game," he said.

Nevada's caucuses, the next contest in the state-by-state process of picking a Republican nominee to oppose Democratic President Barack Obama in the November 6 general election, take place on Saturday.

Romney has won two of the first four nominating contests, taking primaries in New Hampshire and Florida by healthy margins. Gingrich won South Carolina's primary, and former Senator Rick Santorum won Iowa's caucuses.

Trump last year flirted with a presidential run as a Republican. He was derided for pushing a discredited charge that Obama might not have been born in the United States.

He left the Republican Party in December after a presidential debate he had planned to host fell through and has recently hinted that he would mount a third-party White House run if his choice among the Republicans does not win the nomination.