Donald Trump gives a thumbs up after meetings Thursday at Republican National Committee headquarters in Washington. (Jonathan Ernst/Reuters)

Republican presidential candidate Donald Trump met with officials at the Republican National Committee headquarters in Washington on Thursday, two days after he walked back a pledge to support the eventual GOP nominee if he does not win the nomination himself.

“Just had a very nice meeting with @Reince Priebus and the @GOP. Looking forward to bringing the Party together — and it will happen!” Trump tweeted Thursday afternoon.

The GOP front-runner has faced persistent opposition from members of the Republican establishment who fear that his rhetoric about illegal immigration and controversies over how he speaks about women could sink the party’s chances to win the White House in November and threaten its hold on Congress. Anti-Trump conservatives have poured millions of dollars into attack ads throughout the country, hoping to prevent him from clinching the nomination — a plan that rests on stopping him from reaching the 1,237-delegate threshold necessary to declare an outright victory.

Those tensions came to a head Tuesday when, during a televised CNN town hall, Trump said that he no longer intends to uphold a pledge he signed last fall to support the eventual GOP nominee if he is not the winner. Party leaders fear that a third-party run by Trump would rip the GOP apart and sweep the Democratic nominee into office.

The GOP presidential candidates talked about the charges against Donald Trump's campaign manager, Muslims in the U.S. and more, and backed away from past pledges to support whomever becomes the nominee during the CNN town hall on March 29. (Jenny Starrs/The Washington Post)

“No, I don’t anymore,” Trump told CNN’s Anderson Cooper when asked whether he stood by the pledge. Instead, he said, he will wait to see who emerges as the party’s standard-bearer.

“I have been treated very unfairly,” Trump added later.

Trump’s meeting with the RNC on Thursday — which was not publicly disclosed before it occurred — and his subsequent tweet seemed organized to show that the two sides are working together despite Trump's criticism of party leadership.

Trump told Fox News that the meeting with the RNC was productive during a pre-taped appearance on The O’Reilly Factor, and called the party officials “very good people.”

“They are very good people, very actually terrific meeting I think. It’s really a unity meeting,” he said, according to an advance transcript of the interview, which will air in full Thursday evening. “I get along with people, I get along with people very well. We had a very good meeting with the RNC.”

But even though the RNC appears intent upon smoothing things over with Trump, establishment leaders such as 2012 GOP nominee Mitt Romney have continued to question the authenticity of Trump’s conservative principles and have voiced fears that he will irreparably damage the party’s brand. In Wisconsin, which will hold its primary Tuesday, conservative leaders have rallied behind Sen. Ted Cruz (R-Tex.) in hopes of stopping Trump from adding to his delegate lead.

The Fix's Aaron Blake breaks down what's at stake for the GOP candidates in the April 5 Wisconsin primary. (Peter Stevenson/The Washington Post)

Trump has dismissed attacks on his conservative bona fides and temperament, holding up his many victories in nominating contests throughout the country to show that he is what voters want.

“I’m the front-runner by a lot. I’m beating Ted Cruz by millions of votes,” he said. “This was not going to happen with the Republican Party. People who have never voted before, Democrats and independents are pouring in and voting for me.”

Trump also met with several foreign policy advisers while in Washington — a team whose members he has not fully disclosed, but which he said will be unveiled Friday. The meeting, according to his campaign, took place at the Old Post Office Pavilion, which the billionaire is transforming into a Trump-branded hotel.

The real estate mogul has come under fire for refusing to take the use of nuclear weapons off the table as a component of military confrontations. He further raised eyebrows when he suggested that countries like Japan and South Korea may be justified in pursuing the development of their own nuclear weapons in service of their own defense, for which he says the United States bears a disproportionate burden.

He told Fox News Thursday that his foreign policy advisers agreed with him on nuclear power and on his calls to reconfigure NATO, which he said last week is "obsolete."

“A number of them commented that what I said was absolutely correct as to nuclear,” Trump said. “They also felt I was right as to NATO because we are paying a disproportionate share of NATO and NATO is largely obsolete. It’s got to be restructured and it’s got to be changed and other countries have to pay some of the bills.”