Paul Ryan. CNN In a move without precedent in recent political history, House Speaker Paul Ryan on Thursday said he is not ready to support Donald Trump, his party's presumptive presidential nominee.

In a CNN interview, the speaker said that he cannot support or endorse Trump until he unifies the Republican Party.

"To be perfectly candid with you, Jake, I'm just not ready to do that at this point. I'm not there right now," Ryan told CNN host Jake Tapper.

He continued: "I hope to, and I want to. But I think what is required is to unify this party. And I think the bulk of the burden on unifying the party will have to come from our presumptive nominee."

The speaker called on Trump to "set aside bullying, set aside belittlement."

Despite his refusal to endorse Trump, Ryan congratulated the real-estate magnate for effectively clinching the nomination, saying that Republican leaders should take note of the sentiments Trump tapped into. The speaker also said his nonendorsement right now does not mean he is supporting Democratic presidential frontrunner Hillary Clinton.

"No Republican should ever consider supporting Hillary Clinton. Let me make that clear," Ryan said.

Trump fired back in a statement late Thursday, saying he was "not ready to support" Ryan's agenda, either.

"I am not ready to support Speaker Ryan's agenda," he said. Perhaps in the future we can work together and come to an agreement about what is best for the American people. They have been treated so badly for so long that it is about time for politicians to put them first!"

Ryan's stance sets him apart from other congressional leaders, many of whom have slowly begun to coalesce around the controversial real-estate mogul. On Wednesday, Senate Majority Leader Mitch McConnell offered a tepid endorsement of Trump, saying that he was "committed to supporting the nominee."

Still, Trump has faced extraordinary resistance among many prominent Republican Party figures. Both former Presidents George W. Bush and George H.W. Bush declined to endorse Trump, and 2012 presidential nominee Gov. Mitt Romney's staff confirmed on Thursday that he would not attend the Republican National Convention in July.