By Alistair Bell and Steve Holland

WASHINGTON/HAMPTON. N.H. (Reuters) - U.S. presidential hopeful Donald Trump is expected on Thursday to sign a pledge not to run as an independent candidate if he loses the Republican Party nomination, a party official said, despite earlier refusals to rule out a third-party bid.

If he signs, it will be a victory for the party establishment in its efforts to rein in the billionaire, who leads opinion polls but has upset mainstream Republicans with his brash style and unusually personal attacks on rivals.

An independent Trump candidacy in the November, 2016 election could split support for the Republican Party and give the Democrats a huge boost.

Almost two months after reaching the top of opinion polls among Republicans, Trump has come under sustained fire for the first time from one of his main rivals for the nomination, Jeb Bush, the former Florida governor.

Nudging Trump toward signing the pledge was a weeks-long effort by Republican National Committee Chairman Reince Priebus and other officials, an RNC official said.

"This gets the proverbial elephant out of the room and allows the focus to be on winning the White House and taking the prospect of a third party off the table,” the official said.

ABC News reported that Trump was likely to initial the document, but quoted a Trump spokeswoman as saying he had not made up his mind yet.

The real estate magnate and reality television host is due to hold a news conference in New York on Thursday afternoon after meeting the RNC's Priebus.

"THE END IS NEAR"

Reuters/Ipsos polling shows Trump with support among nearly 31 percent of self-identified Republicans as of Sept. 1, with Bush at nearly 12 percent, behind former neurosurgeon Ben Carson. http://bit.ly/1hWTSGh

Bush launched a campaign ad this week ridiculing Trump as a Democrat-in-disguise, signaling a more aggressive stance.

In a campaign speech on Thursday in Hampton, New Hampshire, Bush criticized Trump's plans for drastic measures to stop illegal immigration on the U.S.-Mexican border.

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"Donald Trump’s view is that the end is near. His pessimistic view is that we should close the border,” he said. Bush briefly broke into Spanish as he called for Republicans to “campaign with our arms wide open.”

Bush told ABC's "Good Morning America" TV program he would back Trump if the businessman-turned-politician ended up facing Democrat Hillary Clinton in the 2016 general election. Trump, on the same program, also said he would back Bush over Clinton.

Trump drew boos from the audience at a Republican debate in Ohio last month when he refused to rule out going it alone in 2016.

But he has been under pressure since then to declare his party loyalty, and party officials have been working for weeks to have him sign a pledge. “It reflects a lot of behind the scenes work,” the RNC official said.

Political parties at the state level often ask candidates to sign loyalty pledges, but this is believed to be the first time the Republican Party has done it nationally.

Another of the 17 Republican candidates, New Jersey Governor Chris Christie, urged Trump to cut the "pre-Labor Day Weekend drama" surrounding the pledge.

"Every candidate has to make the decision about do you really want the nomination of our party or do you want to have it both ways," he told Fox News Channel’s "America’s Newsroom" TV show. "This is just a little too much drama. Enough," he said.

Christie then signed the pledge on air.

(Reporting by Alistair Bell in Washington and Steve Holland in Hampton, N.H.; Additional reporting by Alina Selyulkh; Editing by Jeffrey Benkoe)