What once seemed like a way to box in Donald Trump now has boomeranged on the Republican Establishment — the pledge to support the GOP nominee.

Fearing that a defeated Trump might take his populist campaign independent, Republican Party Chairman Reince Priebus offered up the pledge. The 10 candidates who participated in the party’s first primary debate in August all raised their hands when asked if they would sign such a pledge. All, that is, except for Trump, who earned a round of boos.

“What you do is you go to sleep for a couple of days and you wake up and you honor [the pledge].”

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The real estate tycoon eventually did sign the pledge in September, though, and then tweeted a picture of it.

Now that Trump is the presumptive nominee, he wants to know why some of his vanquished primary opponents are not honoring that commitment. He called them “sore losers” at a campaign rally in Maine on Wednesday.

“It was a rough campaign,” he said. “They say it was the roughest campaign ever in the history of Republican politics, but what you do is you go to sleep for a couple of days and you wake up and you honor [the pledge].”

Former Florida Gov. Jeb Bush has refused to endorse Trump and says he’ll skip the Republican National Convention. Gov. Scott Walker, Sen. Ted Cruz, Gov. John Kasich, Sen. Lindsey Graham, and businesswoman Carly Fiorina have all declined to endorse Trump as well.

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Trump went back and forth on whether he would honor the pledge even after he signed it, hinting at various times that it may not apply if he was not treated fairly. But he said Wednesday that he would have honored it had he lost and added that those who are breaking their promise should not be allowed to run for office again. He called the pledge “legally binding.”

GOP spokeswoman Lindsay Walters sidestepped a question about whether the party could or would be inclined to try to enforce the pledge.

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“The Supreme Court is too important to lose for generations and we simply can’t afford to let Hillary Clinton win the White House,” she told LifeZette in an email. “We must unify as a party. There’s too much at stake.”

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Henry Olsen, an elections expert at the Ethics & Public Policy Center, expressed doubt that the pledge is anything more than a political statement.

“There is no contract between two candidates who mutually pledge to support one another; in legal terms, those words are merely statements of intent with no binding effect,” he told LifeZette in an email.

Olsen also questioned how effective highlighting broken pledges would be even as a political talking point.

“I doubt pointing out which presidential candidates are going back on their statements will have much effect,” he wrote. “Voters tend to have low opinions of political promises and I doubt that anyone will be likelier to support or oppose Trump because he points out someone broke their word.”