Speaker Paul Ryan has backed away from his pledge to support whoever becomes the nominee, saying he's "not ready" to endorse Republican presidential candidate Donald Trump. Other GOP heavyweights, including the Bushes, are also not giving endorsements. (Jenny Starrs/The Washington Post)

In an extraordinary rebuke of the Republican Party’s presumptive presidential nominee, House Speaker Paul D. Ryan (Wis.), the nation’s highest-ranking GOP official, said Thursday that he could not support Donald Trump until he changes his tone and demonstrates that he shares the party’s values.

While acknowledging that Trump has mobilized a powerful grass-roots movement and earned the nomination, Ryan said that Trump has not shown himself to be “a standard-bearer who bears our standard” — and he put the onus on the business mogul to recalibrate his campaign and offer a more inclusive vision.

Asked by CNN anchor Jake Tapper whether he backs Trump, Ryan responded: “I’m just not ready to do that at this point. I’m not there right now. And I hope to, though, and I want to. But I think what is required is that we unify the party. And I think the bulk of the burden on unifying the party will have to come from our presumptive nominee.”

“This is the party of Lincoln, of Reagan, of Jack Kemp. And we don’t always nominate a Lincoln and a Reagan every four years, but we hope that our nominee aspires to be Lincoln- and Reagan-esque,” Ryan said, adding that he hopes the candidate “advances the principles of our party and appeals to a wide, vast majority of Americans.”

Ryan’s comments, which came as a surprise to some close allies, deepened the divide in a party now facing a painful reckoning about Trump. The GOP’s only two living presidents — George H.W. Bush and George W. Bush — said they would not endorse him, while its past two nominees — Mitt Romney and John McCain — said they did not plan to attend Trump’s nominating convention this summer in Cleveland. McCain, however, said he would support Trump and has offered to counsel him on foreign policy.

Trump was defiant in his response to Ryan, offering a firm defense of his candidacy and asserting that he has a mandate from Republican voters. In a notable departure from his handling of previous feuds, Trump did not insult Ryan personally.

“I am not ready to support Speaker Ryan’s agenda,” Trump said in a statement. “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 remarks broke a previous pledge to support whoever becomes the GOP nominee. It also puts him at odds with Senate Majority Leader Mitch McConnell (R-Ky.), who offered tempered support for Trump on Wednesday, and Republican National Committee Chairman Reince Priebus, a Ryan friend who has urged Republicans to unite behind Trump.

Priebus is trying to broker a Trump and Ryan meeting next week. Ryan spokesman Brendan Buck tweeted that the speaker would be “happy to attend.”

New Jersey Gov. Chris Christie (R), a Trump supporter and adviser, told reporters in Trenton that he would reach out to Ryan to discuss his concerns.

Other Republicans are swiftly coming around on Trump. Former Texas governor Rick Perry, who delivered the first vicious takedown of Trump last summer, told CNN that he now supports him and is open to being his running mate. Nebraska Gov. Pete Ricketts, whose family helped bankroll an anti-Trump super PAC, plans to endorse Trump on Friday at a rally in Omaha.

The Republican presidential front-runner reversed course on a whole load of issues – all on May 4. (Peter Stevenson/The Washington Post)

The tensions between Trump and Ryan go beyond temperament. They have philosophical differences about the size and scope of government. Ryan champions free-trade agreements, international military engagement, and sweeping overhauls of Social Security and Medicare, whereas Trump is an avowed opponent of recent trade deals, foreign interventions and proposed changes to entitlement programs.

Furthermore, Ryan frames his politics in stark moral terms, while Trump’s manner was forged by his experiences in the Manhattan business and tabloid wars of the 1980s.

“It’s time to go from tapping anger to channeling that anger into solutions,” Ryan said. “It’s time to set aside bullying, to set aside belittlement and appeal to higher aspirations, appeal to what is good in us and to lead a country and a party to having a vast majority of Americans enthusiastic about choosing a path.”

Ryan said that no Republican should consider supporting likely Democratic nominee Hillary Clinton, but he did not spell out what he would do if he could not get around to backing Trump. Ryan’s indecision comes as some conservatives, including freshman Sen. Ben Sasse (R-Neb.), are trying to draft an independent, third-party candidate.

“It’s a moment of moral clarity,” said Peter Wehner, a center-right commentator and senior fellow at the Ethics and Public Policy Center. “If Trump is smart, he’ll take this message to heart and figure out that he doesn’t have a snowball’s chance in hell of winning if he doesn’t unify the fractured party.”

Trump is expected to visit Washington next week to meet with lawmakers. But there are no plans for Trump to address the full House Republican Conference — a departure from tradition for both parties, in which the presumptive nominees trek to Capitol Hill to meet with their respective cau­cuses in meetings hosted by the congressional leadership.

Aides to McConnell declined Thursday to say whether any such meeting with Trump would occur among Senate Republicans.

Trump’s supporters in Congress shrugged off Ryan. “Republicans, we have to listen to the voters’ voices,” said Rep. Lou Barletta (R-Pa.). “Everyone will come at their own time. We’re not pressuring anyone, just welcoming.”

Polling shows Trump is deeply unpopular with wide swaths of the electorate — especially women, young people, Latinos and African Americans — and Republicans are fearful that November could be a bloodbath that jeopardizes their Senate and House majorities.

In part because of his role as chairman of the Republican National Convention, Ryan has refrained from criticizing Trump too directly. But he has made no secret of his disdain for him. He has given a series of speeches outlining a distinct policy agenda and message as a sort of road map for embattled GOP lawmakers who may seek to differentiate their candidacies from Trump.

Ryan’s comments Thursday offer a new way for like-minded Republicans to address Trump’s pending nomination.

“There has been growing anxiety among members in purple and blue districts, marginal seats,” said Rep. Peter T. King (R-N.Y.), a Ryan ally. “Paul truly believes what he’s saying. . . . It’s personal and sincere. But there is a political equation to all this. He knows what the feeling is inside of the House as much as anyone.”

Ryan shocked some leading Republicans, who expected he would dutifully line up behind the presumptive nominee. William J. Bennett, a former Reagan administration official and a mentor to Ryan, said he was “knocked out of my chair” as he watched Ryan on CNN.

“This is a slap at the people,” Bennett said. “He thinks he can nudge Trump in a certain direction, but it doesn’t make sense to expect Trump to have some kind of personality transformation. His approach was not conducive to unification, which is what the party needs.”

Former House speaker Newt Gingrich, a Trump defender, said the GOP is undergoing “a natural process” but was critical of Ryan.

“Ryan has to bring together a House GOP in which more and more members will support Trump,” Gingrich said. “McConnell and McCain have been far better leaders in trying to bring the party together.”

Ryan made his decision about Trump on Wednesday, according to a senior House leadership aide, who spoke on the condition of anonymity to speak freely about Ryan’s internal deliberations.

It’s unclear if the speaker ever will fully embrace Trump, but Ryan still plans to oversee the convention as co-chairman, a largely ceremonial role. He does not plan to deliver a formal speech in Cleveland and expects to focus fully on campaigning for House Republicans, the aide said.

“Ryan is a conviction politician,” said Wehner, an ally. “He’s not a Republican first and foremost and only. Ryan is somebody who has a set of convictions and whose philosophical beliefs transcend even the party beliefs, and that’s not true for everybody else.”