Donald Trump endorses Mitt Romney
Romney also turned down an invitation to participate in a presidential debate that Trump planned to moderate in Iowa in December.
Trump canceled the debate after all the candidates except Gingrich and Santorum refused to participate.
Trump has played an unusually prominent role in the presidential contest since last spring, when he mused publicly about joining the Republican field.
His blunt criticism of President Barack Obama and fierce warnings of a nation in decline resonated with tea party activists. At one point, polls showed him briefly surging to the top of the field.
Trump stirred controversy and considerable criticism during that time by openly questioning the validity of Obama's birth certificate, lending credence to the chorus of "birthers" who believe Obama was not born in the United States and thus is ineligible to be president.
The fuss pushed Obama to release a long-form version of his birth certificate, proving he was born in Hawaii in 1961.
The president dismissed Trump as a "carnival barker" for ginning up the issue and then memorably skewered his nemesis at the White House Correspondents' Association dinner in Washington, which Trump attended.
Trump announced last May that he would not be a candidate for the GOP nomination. But he welcomed other hopefuls to his office at Trump Tower on Manhattan's Fifth Avenue for strategy sessions.
Romney made the trek, as did former candidates Michele Bachmann, Herman Cain and Rick Perry. Trump even took Sarah Palin out for a widely publicized pizza dinner in Times Square when she was considering a presidential bid.
But the 2008 GOP vice presidential nominee ultimately decided against running. Gingrich visited Trump in December when he was topping polls in Iowa and nationally.
After the meeting, Gingrich told reporters he had persuaded Trump to mentor a group of children from some of New York's poorest schools.
The gesture came after Gingrich was criticized for suggesting that poor youngsters should do janitorial chores in their schools to learn the importance of work.
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