"I signed the pledge, it's why you have to be careful with pledges you sign, that I would support the Republican nominee," Kasich answered when asked a question by the audience after a speech Wednesday at the Council on Foreign Relations.

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"Is it possible that you change your mind?" he asked rhetorically. "Yeah. It takes something extreme to do it."

Kasich immediately cast aside the notion of Trump winning the White House in his next breath.

"I will tell you sir, there's no way that Donald Trump is going to be president. I've been saying that for weeks. I don't even take it seriously because he isn't going to win. It's not going to happen," he added.

The vast majority of the GOP presidential candidates, including Kasich, have castigated Trump after his Monday call to halt all Muslim immigration to America amid concerns for national security. It's become the standard refrain from Kasich, whose campaign and allied super-PAC have launched an onslaught of attacks targeting Trump.

Former Florida Gov. Jeb Bush told MSNBC's "MTP Daily" that he will support the party's nominee , but cast aside the question as moot because he believes Trump won't win. Republican National Committee Chairman Reince Priebus, who told Washington Examiner he disagreed with Trump's plan, added that "we need to aggressively take on radical Islamic terrorism but not at the expense of our American values." But Kasich is the only Republican candidate to even publicly consider the notion of not backing Trump if he wins the primary. Sen. Ted Cruz (Texas) told reporters Tuesday that he would "absolutely" support the GOP nominee, while Sen. Rand Paul (Ky.) told Yahoo News that he would do so despite not being "enthusiastic" about it.

It was Priebus who directed all of the candidates to sign a pledge to support the eventual nominee — a pledge aimed directly at Trump — earlier this year, with Priebus traveling to Trump Tower to meet with Trump hours before he signed.