RNC Chair Reince Priebus on this Sunday's Face the Nation with a not so thinly veiled threat for any of Trump's former primary opponents who aren't willing to get on board and endorse him for president.

Republican National Committee Chairman Reince Priebus said Sunday that it’s high time for Donald Trump’s former primary challengers to come on board and support his campaign—and suggested there could be trouble for them in 2020 or 2024 if they don’t.

“Those people need to get on board,” he told CBS’ “Face the Nation.” “And if they’re thinking they’re going to run again someday, 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.”

Several of Trump’s former Republican primary opponents, including Ohio Gov. John Kasich, Texas Sen. Ted Cruz, and former Florida Gov. Jeb Bush have not endorsed Trump in the months since the GOP businessman secured the nomination. Asked explicitly whether that meant there would be penalties for the handful of 2016 Republican hopefuls who have not endorsed Trump if they opted to run again in 2020 or 2024, Priebus said nothing has been decided but that it’s something the party will “look at.”

All of the major Republican candidates, including Trump, signed a so-called “loyalty pledge” last summer stating that they would support the eventual Republican nominee “regardless of who it is.” The document wasn’t legally binding, but candidates were asked to sign it last summer after Trump at the time wouldn’t rule out a third-party bid if he lost the nomination.

“People in our party are talking about what we’re going to do about this. I mean there’s 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,” Priebus said. “So what’s the penalty for that? It’s not a threat, but that’s just the 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, and those participants don’t follow through with the promises that they made in that process, what-- what should a private party do about that if those same people come around in four or eight years?” Priebus continued.