WASHINGTON — The Republican hierarchy is challenging each of the party’s presidential candidates to sign a pledge not to undertake a third-party bid — a move aimed squarely at Donald Trump.

The Republican National Committee’s challenge was confirmed by multiple campaigns, and the timing of the pledge suggests an agreement has been reached.

Trump, who is leading the packed field in early polls, has threatened to mount a third-party bid if he did not win the nomination. Such a decision would make it even more difficult for the GOP to take back the White House in 2016.

RNC officials have been working with Trump’s campaign to avoid such a scenario.

In recent days, Trump has hinted that the lobbying was beginning to work. “I think a lot of people are going to be very happy,” he said Saturday in Tennessee.

The Trump campaign did not respond to questions about the pledge late Wednesday.

The RNC’s chairman, Reince Priebus, planned to meet with Trump at his campaign headquarters in New York City on Thursday shortly before Trump was to speak to reporters.

The meeting was confirmed by two RNC officials who weren’t authorized to discuss the plan publicly and requested anonymity.

Several candidates confirmed that they would sign the pledge, among them Ohio Gov. John Kasich and former Florida Gov. Jeb Bush, although few doubted the intentions of the vast majority of the GOP’s 17 presidential contenders.

Sen. Rand Paul of Kentucky said Thursday on “CBS This Morning” that he was likely to sign, too, adding that he was “sympathetic’ with third-party efforts, believing they spur the two major parties to act more in the public interest.

The RNC’s pledge asks candidates to promise to “endorse the 2016 Republican presidential nominee regardless of who it is.”

Further, it asks them to 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.”

An RNC spokesman declined comment.