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New York (CNN)Donald Trump has signed the pledge.

The Republican presidential front-runner met privately with Republican National Committee Chairman Reince Priebus Thursday afternoon, and soon after, came out to the lobby of Trump Tower to declare that he has signed a loyalty pledge. This means Trump has promised to support the party's eventual nominee -- whoever that may be -- and that he will not run as a third-party candidate.

"The best way for the Republicans to win is if I win the nomination and go directly against whoever they happen to put up. And for that reason, I have signed the pledge," Trump said, holding up the paper. "So I will be totally pledging my allegiance to the Republican Party and for the conservative principles for which it stands."

He added: "We will go out and fight hard, and we will win."

All 17 Republican presidential candidates have now pledged to support the GOP's eventual presidential nominee, Priebus announced Thursday evening in a statement, billing it as a sign of "party unity."

But if Trump's official declaration of allegiance to the party serves to calm the nerves of establishment Republicans -- at least for now -- it could also invite backlash from some of the bombastic candidate's die-hard supporters.

Trump has propelled himself to the top of the polls by casting himself as an anti-establishment, outsider candidate, railing against career politicians and the Washington political class.

Signing an RNC pledge complicates that image.

Katrina Pierson, a spokeswoman for the Tea Party Leadership Fund and a Trump defender, told CNN she personally does not condone the pledge.

"The GOP has not been loyal to members of its own party during previous election cycles," PIerson said. "I can't see any reason why he would give up that leverage considering a lot of his supporters like the idea that he's running against the establishment."

Thursday's 15-minute sit-down with Priebus comes amid unease about whether the billionaire businessman would rebuff the party and seek the White House as an independent. Soon after Trump announced his candidacy, Priebus asked the real estate magnate to tone down his fiery rhetoric on immigration, as establishment Republicans grew increasingly worried that Trump was angering the Hispanic community.

Trump explained Thursday that he came to the decision to sign the pledge because the Republican Party in recent months has been "extremely fair" to him.

"The RNC has been absolutely terrific over the last two month period and as you know, that's what I've wanted," Trump said. "I don't want to be treated any differently."

Asked what he got in return for signing the paper, Trump responded: "assurance that I will be treated fairly."

RNC officials began circulating a pledge to various GOP presidential campaigns this week, measuring up how much appetite there is in the field to commit to supporting the eventual nominee.

"I, ________, affirm that if I do not win the 2016 Republican nomination for President of the United States I will endorse the 2016 Republican presidential nominee regardless of who it is," it reads.

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The pledge continues: "I further pledge that I will not seek to run as an independent or write-in candidate nor will I seek or accept the nomination for president of any other party."

Advisers to the candidate have said all along that Trump was never seriously interested in launching an independent run, which is an arduous -- and costly -- process.

The pledge has not only put pressure on Trump to commit to the party, it's also forcing some of his rivals to promise to support Trump if he were to clinch the GOP nomination.

It's a particularly uncomfortable position for a candidate like Jeb Bush, who in recent weeks has publicly clashed with Trump. The two men have released attack videos on social media, and openly criticized one another on the trail.

On ABC's "Good Morning America" Thursday morning, Bush lashed out at Trump, saying, "I think Donald Trump is trying to insult his way to the presidency and it's not going to work."

However, pressed on whether he would support Trump if he were to become the nominee, the former Florida governor answered in the affirmative.

"Yes, I would, of course. We need to be unified. We need to win," Bush said.

After Trump's press conference, Bush tweeted a tongue-in-cheek version of the pledge that said, "Voted Republican since 1972."

Meanwhile, others are raising questions about just how enforceable a loyalty pledge is.

"You're right, it's unenforceable," said Carly Fiorina on CNN's "New Day." "It is, more than anything else, your word."

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