Donald Trump signed a pledge Thursday promising not to mount a third-party candidacy if he loses his campaign for the Republican presidential nomination, a move urged by the party.

The billionaire developer agreed to the pledge, which is not legally binding, after meeting privately with Republican National Committee chairman Reince Priebus, who has asked all 17 GOP contenders to do likewise.

Addressing a news conference in the lobby of his Trump Tower in midtown, Trump held the paper aloft after saying, "I will be totally pledging my allegiance to the Republican Party and the conservative principles for which it stands."

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Trump had resisted making such a promise during the previous GOP debate. Party officials have worried that a third-party challenge could siphon votes from the eventual GOP nominee and jeopardize efforts to retake the White House. But Trump faced losing his spot on primary ballots in some states if he refused to pledge fidelity to the party.

Citing his lead atop nearly all polls, Trump said of the Democrats, "the best way for the Republicans to win is if I win the nomination and go directly against whoever they happen to put up."

Trump's wide-ranging news conference covered now-familiar ground -- criticizing the country's direction under Barack Obama and ridiculing GOP rival Jeb Bush, the former Florida governor, as "low energy" and "sad." He also approved of a federal judge's Deflategate ruling that favored Tom Brady ("He's a great guy," Trump said of the quarterback) and of rapper Kanye West, who told an awards show audience last weekend that he wants to run for president in 2020. Said Trump: "He loves Trump!"

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To his left as he stood at a lectern were young women at Trump's store selling eponymous tennis balls, fragrances, hats, teddy bears and more.

The pledge circulated by the RNC also calls on candidates to back whoever wins the GOP nod. Trump said repeatedly that he doesn't foresee breaking his word, but he didn't absolutely rule it out.

"I see no circumstances under which I would tear up that pledge," he said.

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Several others have already signed the pledge or signaled they would, including Bush, Ohio Gov. John Kasich and New Jersey Gov. Chris Christie.

Trump was once considered a long shot, but his aggressive rhetoric on illegal immigration and foreign policy, and his belittling of the political establishment have catapulted him to the front of a crowded field.

Trump has said he'll build a wall on the border with Mexico, deport millions of people who are illegally living in the United States, seize Mideast oil and "make America great again."

In a nationwide Monmouth University poll released Thursday, Trump expanded his already solid lead over all the other candidates. He had the support of 30 percent of GOP or GOP-leaning voters. Retired neurosurgeon Ben Carson was second with 18 percent.

With AP