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It's official: Donald Trump's presidential campaign just took a right turn.

The 69-year-old Republican candidate signed a loyalty pledge to the Republican Party on Thursday, ruling out a third-party or independent run if he loses the Republican Party nomination.

"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," he said at a press conference at his Trump Tower in New York City, holding up the document. So I will be totally pledging my allegiance to the Republican Party and the conservative principles for which it stands and we will go out and we will fight hard and we will win."

"We will win," he said. "And most importantly, we will make our country great again, because that's what it's all about. We have to make our country great again."

READ: Donald Trump clarifies Megyn Kelly "blood" comment and adds, "Only a deviant would think anything else"

Donald Trump

Spencer Platt/Getty Images

The document states, "I, Donald Trump, 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. 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.

In statement, Trump also said, "This is far and away the best way to secure victory against the Democrats in November 2016. I am leading in all local and national polls—my whole life has been about winning and this is what must be done in order to win the election and, most importantly, to Make America Great Again!"

Trump has been beating his fellow GOP candidates in the polls since he announced his presidential bid in June but had also sparked concern about the level of his devotion to the Republican party, with some critics citing his past support of and friendship with Democratic candidate Hillary Clinton, who had attended his 2005 wedding to third wife Melania with her husband and former President Bill Clinton.

Trump had donated at least $10,800 to Hillary's presidential and Senate election campaigns between 2002 and 2008, records show. He supported Republican leader Mitt Romney to win the 2012 election.

Before signing the Republican loyalty pledge, Trump met privately with Republican National Committee Chairman Reince Priebus, NBC News reported, adding that the RNC had recently reached out to other Republican candidates to gauge their support for such a pledge.

At the press conference, Trump was also asked about his feelings about Kanye West's declaration at the 2015 MTV VMAs that he will run for president in 2020. The Republican candidate had told Rolling Stone recently that the rapper "is actually a different kind of person than people think. He's a nice guy" and that he hopes to "run against him someday."

Trump told reporters on Thursday he will "never say bad" about West because "he loves Trump." The rapper has not commented.

"Kanye West, I love him," Trump said. "Maybe in a few years, I'll have to run against him. I don't know. I'll take that back. He's been so nice to me. You people have sort of seen—because I've been a counter puncher. I only hit people when they hit me. And Kanye West is so great. I would never say bad things about him because he says such nice things about me."

—Additional reporting by Baker Machado

(E!, MSNBC and NBC News are part of the NBCUniversal family.)