Several high-profile Republicans have indicated recently that they can't support Donald Trump for president.  The two President Bushes, Mitt Romney, much of the GOP foreign policy establishment, and several senators, including Ben Sasse, have all said they will sit out the election.

Speaker Paul Ryan hasn't quite abjured his support for Trump, but he made clear to Jake Tapper that Trump must change for him to endorse him.

"I'm just not ready to do that at this point. I'm not there right now," the Wisconsin Republican told CNN's "The Lead with Jake Tapper" in an interview.

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. His move gives down-ballot Republicans cover to hold off on supporting Trump. It could also keep his agenda in the House from being overtaken by Trump's policy positions.

Ryan said 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 responded to Ryan late Thursday afternoon, saying in a statement that "I am not ready to support Speaker Ryan's agenda. 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!"

Trump didn't address Ryan's comments at a campaign event Thursday night in Charleston, West Virginia. But speaking to reporters at the event, Trump's campaign manager, Corey Lewandowski, dismissed the idea that the remarks are a slap in the face to the presumptive nominee coming from the leader of the Republican Party.

Lewandowski then pointed to onstage to Trump.

"That's the leader of the Republican Party," Lewandowski said, adding that it's too early to say where Trump and Ryan might find common ground in terms of their political agenda.

Trump was surprisingly diplomatic about the snub from Ryan, considering that the speaker is asking him to toss out the agenda that got him the nomination and substitute Ryan's.

But perhaps Ryan would be satisfied with a change in tone from Trump.  Other politicians like Senator Susan Collins have indicated they could support Trump if he stopped his juvenile name-calling and insults.  That's easier said than done for Trump, who appears to relish getting laughs and cheers at his rallies when he lays into his latest target.

Calling on Trump to unify the party before he offers his support, Ryan has to realize that things have gotten way beyond simple concepts like "unity."  For many in the GOP, opposing Trump has become a matter of principle, which is why, like the Whigs in the 1840s, the GOP as we know it is headed for the dustbin of history.