Paul Ryan
Paul RyanReuters

Despite the fact that Donald Trump is well on his way to becoming the Republican presidential nominee, House Speaker Paul Ryan is not yet ready to support him.

"I'm just not ready to do that at this point. I'm not there right now," the Wisconsin Republican told CNN in an interview on Thursday.

Ryan's position makes him the highest-level GOP official to reject Trump since the real estate mogul became the last candidate standing in the party's nominating contest.

He said in the interview he hopes to eventually back Trump and "to be a part of this unifying process." The first moves, though, must come from Trump, he said.

Ryan said he wants Trump to unify "all wings of the Republican Party and the conservative movement" and then run a campaign that will allow Americans to "have something that they're proud to support and proud to be a part of."

"And we've got a ways to go from here to there," Ryan said.

Asked whether Trump's proposed Muslim ban, his opposition to free trade and his call to deport 12 million undocumented immigrants would preclude him from ever supporting Trump, Ryan said, "We got work to do."

Trump became the presumptive Republican nominee after his two remaining rivals, Ted Cruz and John Kasich, both suspended their campaigns on Tuesday and Wednesday, respectively.

Their moves came after they lost the Indiana primary to Trump.

Ryan's comments to CNN were striking because Senate Majority Leader Mitch McConnell said Wednesday night that he'd back Trump.

The House speaker said he'd only started considering whether he'd support Trump after the real estate mogul won Indiana's primary Tuesday.

"I thought about this two days ago. I thought, actually, this thing was going to go to June 7 at the very least -- probably to a convention -- and so this is all pretty new for us," he told CNN.

"The bulk of the burden on unifying the party will have to come from our presumptive nominee," Ryan continued. "I don't want to underplay what he accomplished. ... But he also inherits something very special, that's very special to a lot of us. This is the party of Lincoln and Reagan and Jack Kemp. And we don't always nominate a Lincoln or a Reagan every four years, but we hope that our nominee aspires to be Lincoln- or Reagan-esque -- that that person advances the principles of our party and appeals to a wide, vast majority of Americans."

"And so,” he said, “I think what is necessary to make this work, for this to unify, is to actually take our principles and advance them. And that's what we want to see. Saying we're unified doesn't in and of itself unify us, but actually taking the principles that we all believe in, showing that there's a dedication to those, and running a principled campaign that Republicans can be proud about and that can actually appeal to a majority of Americans -- that, to me, is what it takes to unify this party."

Ryan has expressed misgivings about Trump's campaign for months.

When Trump proposed indefinitely banning Muslims from the United States over security concerns in December, Ryan responded that such a move is "not who we are as a party" and in violation of the Constitution.