The Republican National Committee on Wednesday began reaching out to several GOP presidential campaigns, asking if their candidates would sign a pledge not to run as an independent in 2016, multiple campaign sources told Fox News.

“The RNC reached out to us today about signing the pledge in order not to run as an independent,” said one source with a leading GOP presidential campaign.

RNC sources told Fox News that the plan has been in the works for weeks.

The pledge states in part that if the GOP contender does not become the nominee: “I will endorse the 2016 Republican presidential nominee regardless of who it is," and "I will not seek to run as an independent or write-in candidate nor will I seek or accept the nomination for president of another party.”

The pledge follows concerns that leading Republican candidate Donald Trump will run as a third-party candidate if he fails to win the GOP nomination. Such a move would likely result in Trump taking enough votes with him to possibly sink the Republican Party’s chances of defeating the Democratic nominee and taking the White House.

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A GOP source with direct knowledge of the meeting told Fox News that the pledge would be at the center of a scheduled meeting Thursday between Trump and RNC Chairman Reince Priebus in New York City. Trump was scheduled to hold a press availability Thursday afternoon.

During the Aug. 6 Republican debate, Fox News moderator Bret Baier asked the main-stage candidates to raise their hand if they would not pledge to support the eventual nominee and not run as an independent if they lost the nomination. Trump was the only candidate to raise his hand, to a smattering of boos from the Cleveland crowd.

The Associated Press reported that RNC officials had been working privately with Trump's campaign for several weeks to avert the possibility of Trump making a third-party run. In recent days, Trump has suggested he would soon decide whether to rule out an independent campaign.

"I think a lot of people are going to be very happy," he said Saturday in Nashville.

Late Wednesday, a spokesperson for Ohio Gov. John Kasich confirmed that Kasich would sign the pledge, making him the first GOP candidate to go on record that he would do so. Former New York Gov. George Pataki's campaign confirmed to Fox late Wednesday that he had signed the pledge, while former Florida Gov. Jeb Bush did the same when asked by the Associated Press.

The pledge also follows Virginia and South Carolina GOP state parties making a similar pledge effort.

Fox News' Jessica O'Hara and Patrick Manning and the Associated Press contributed to this report.