Donald Trump may be the presumptive Republican nominee for president, but he does not yet have the support of the party’s highest-ranking elected official.

House Speaker Paul Ryan said in an interview on CNN Thursday afternoon that he was not ready to endorse Trump, and called on the businessman to unify the GOP and embrace “conservative principles” in order to earn his support. “To be perfectly candid, I’m not ready to do that right now,” Ryan told CNN’s Jake Tapper when he asked if he would support Trump. It was a line he repeated in various forms during the 10-minute interview.

Ryan’s unwillingness to endorse Trump is a fairly stunning turn in the presidential race, and it comes after the speaker insisted for months that because he will serve as the chairman of the Republican National Convention, he was duty-bound to support whoever the party nominated for president. He did so even as he criticized Trump’s proposals to ban Muslims from entering the United States and for his refusal to condemn the eruptions of violence at his rallies.

“I think he needs to do more to unify this party,” Ryan said on Thursday. “Saying we’re unified doesn’t in and of itself unify us.”

“It’s time to set aside bullying, set aside belittlement and appeal to higher aspirations,” he added.

Ryan has become increasingly visible in recent months as Trump closed in on the nomination, trying to serve as a more substantive, optimistic counterweight as the leader of House Republicans. His effort to raise his profile stoked speculation that he would be drafted at an open convention, but Ryan quashed those reports by declaring that he would not accept the party’s nomination. He didn’t revisit that debate on Thursday, but he voiced concerns that Republicans would not “have a standard-bearer that bears our standards.”

“Conservatives want to know, does he share our values?” Ryan said. “There are lots of questions conservatives are going to want answers to, myself included.” Trump’s nomination represents a clear threat to Ryan’s vision of enacting a conservative agenda in Washington. He has criticized proposals that Ryan has developed to overhaul Medicare and Medicaid and shrink the size of government. The two are also at odds on immigration and trade policy, among other issues. And on Wednesday, Trump appeared to walk back his opposition to an increase in the minimum wage, opening another possible divide with Ryan’s brand of conservatism.

Reince Priebus, the chairman of the Republican National Committee, has already declared Trump to be the presumptive nominee. On Wednesday, Senate Majority Leader Mitch McConnell issued a tepid endorsement of Trump while saying he had “the opportunity and the obligation to unify our party around our goals.”