Republican National Committee Chairman Reince Priebus said Sunday that GOP candidates who ran against Donald Trump and lost may face penalties if they don’t endorse Trump but try to take another shot at the presidency later.

“Those people need to get on board,” Preibus said of Trump’s primary challengers.

Former Florida Gov. Jeb Bush, Ohio Gov. John Kasich and Texas Sen. Ted Cruz have yet to endorse Trump, despite the real estate mogul securing the nomination in July.

“If they’re thinking they’re going to run again some day, I think that we’re going to evaluate the process of the nomination process, and I don’t think it’s going to be that easy for them,” Priebus told CBS’s “Face the Nation.”

Priebus added that those candidates “gave [the RNC] their word” and used information provided by the party.

He said that the process requires candidates to endorse the nominee if they want to be on the ballot in certain states, which could complicate matters in 2020 or 2024.

“People in our party are talking about what we’re going to do about this,” Priebus said. “There is a ballot access issue in South Carolina. In order to be on the ballot in South Carolina, you actually have to pledge your support to the nominee, no matter who that person is. What’s the penalty for that? It’s not a threat. It’s just a question that we have a process in place.”

“And if a private entity puts forward a process and has agreement with the participants in that process,” he continued, “then those participants don’t follow through with the promises that they made in that process, what should a private party do about that if those same people come around in four or eight years?”

Editor’s note: Donald Trump regularly incites political violence and is a serial liar, rampant xenophobe, racist, misogynist and birther who has repeatedly pledged to ban all Muslims — 1.6 billion members of an entire religion — from entering the U.S.