Jeb Bush in Manchester, N.H., last April. (Jabin Botsford/The Washington Post)

New rifts emerged Friday in the already shaky relationship between Republican leaders and presumptive presidential nominee Donald Trump, heightening concerns that the party is headed into a long period of civil war that imperils its chances in the November elections.

A day after House Speaker Paul D. Ryan of Wisconsin took the unusual step of refusing to support Trump, a steady list of other GOP notables joined in the opposition, including former Florida governor Jeb Bush, Sen. Lindsey O. Graham of South Carolina and 2012 presidential nominee Mitt Romney.

Trump, in turn, attacked Ryan and others for refusing to back him, even as he agreed to meet with Ryan next week to air out their differences.

And at the White House, President Obama waded into the opposition’s turmoil for the first time since Trump effectively clinched the nomination, listing concerns about the mogul that he said Republican voters must seriously consider.

For Trump and GOP elites, it amounted to another awkward chapter in their uneasy alliance — not the celebratory moment many had hoped would arrive when the GOP contest was settled.

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)

Trump’s outsider candidacy and outsize persona — and his extreme positions on issues including immigration and Islam — have alarmed broad swaths of the GOP establishment that fear the party is headed for a wipeout in the fall if Trump is not contained or kept at arm’s length. For his part, Trump argues that his brash campaign triumphed over the rest of the GOP field fair and square, and he has suggested he is unlikely to budge on his positions to please Republican leaders.

“A poker game,” said former Republican National Committee chairman Michael Steele, summing up the turbulence. “Everybody wants to see how much they can move the other [side] closer to where they are.”

Amid the upheaval, there was at least one encouraging sign for Republicans hoping that the party comes together: Trump and Ryan agreed to sit down face-to-face.

“Having both said we need to unify the party, Speaker Ryan has invited Donald Trump to meet with members of the House Republican leadership in Washington on Thursday morning to begin a discussion about the kind of Republican principles and ideas that can win the support of the American people this November,” Ryan’s political office said in a statement. Ryan, Trump and RNC Chairman Reince Priebus will also meet separately, the statement said. Ryan’s office said that meeting will also be Thursday.

But Trump was tepid in his own statement on the planned gathering, saying he had agreed to talk to Ryan “before we go our separate ways.”

“So I guess the meeting will take place and who knows what will happen,” he said.

Bush, whom Trump disparagingly labeled “low energy” when they faced each other as presidential candidates, posted a message on Facebook on Friday afternoon saying that he would not support Trump or Democratic front-runner Hillary Clinton.

“He has not displayed a respect for the Constitution,” Bush said of Trump. “And, he is not a consistent conservative.”

Graham, another former presidential candidate, had backed Bush and later Sen. Ted Cruz of Texas in hopes of stopping Trump. He released a statement Friday saying that he “absolutely will not support Hillary Clinton” but “cannot in good conscience support Donald Trump.”

“I do not believe he is a reliable Republican conservative nor has he displayed the judgment and temperament to serve as Commander in Chief,” the statement said.

Former senator and Republican presidential candidate Robert J. Dole, meanwhile, said he would attend the Republican National Convention this summer to back Trump’s nomination.

According to the Washington Examiner, Romney, who plans to skip the convention, said at an awards gala Thursday evening, “I don’t intend on supporting either of the major-party candidates.” William Kristol, a leading conservative commentator and editor of the Weekly Standard, said he met with Romney on Thursday to urge him to consider an independent bid for president.

Some Republicans have gone further, saying they plan to back Clinton over Trump or are at least considering it. Fielding questions from reporters Friday, Obama said Republicans will have to do soul-searching when it comes to the presumptive GOP nominee.

“Not just Republican officials but, more importantly, Republican voters are going to have to make a decision as to whether this is the guy who speaks for them and represents their values,” Obama said. “Republican women voters are going to have to decide, ‘Is that the guy I feel comfortable with in representing me and what I care about?’ ”

The president also suggested Trump is not up to the momentous challenges the country faces.

“I just want to emphasize the degree to which we are in serious times and this is a really serious job,” Obama said. “This is not entertainment. This is not a reality show.”

Secretary of State John F. Kerry joined in the Trump criticism Friday while delivering the commencement address at Northeastern University in Boston. Kerry noted the diversity of the crowd — “classmates of every race, religion, gender, shape, size” — before saying, “In other words, you are Donald Trump’s worst nightmare.”

Speaking to journalist Mike ­Allen at a Politico Playbook Breakfast on Friday morning, Priebus — who is close with Ryan and has called for the party to rally behind Trump — said he believed the departure of Cruz and Ohio Gov. John Kasich from the race this week took Ryan by surprise and forced him to confront the prospect of Trump as the presumptive nominee sooner than anticipated. The RNC chairman predicted that Ryan will ultimately come around.

“I think he’s going to get there,” Priebus said. “By the way, he wants to get there. It’s just that he wants some time to work through it.”

Ryan departed from his previous pledge to support the GOP nominee when he told CNN’s Jake Tapper on Thursday that he is “not there right now” with Trump.A senior Ryan aide, speaking on the condition of anonymity, said Ryan has been particularly troubled by Trump’s controversial statements on the trail that single out certain groups, including his call for a ban on all Muslims entering the United States after December’s terrorist attacks in San Bernardino, Calif.

Priebus said Trump called him within minutes of Ryan’s interview and solicited advice on how to proceed.

“It wasn’t like furious or anything, it was just like, ‘What do I need to do?’ ” Priebus said. “And so I said, ‘Listen — my view is just relax and be gracious, and I will talk to Paul and we will try to work on this.’ ”

But Trump campaign manager Corey Lewandowski disputed that account. “Mr. Trump didn’t ask what he needed to do,” he said.

In his statement, Trump said, “I told Reince that I thought it was totally inappropriate what Paul Ryan said and thought it was good for me politically.”

On Friday, Trump ripped into Ryan and others on Twitter and at a rally in an airplane hangar in Omaha. where he picked up the endorsement of Nebraska Gov. Pete Ricketts (R), whose parents collectively gave $5.5 million to an anti-Trump super PAC.

“Paul Ryan, I don’t know what happened. I don’t know,” Trump said as the crowd booed the House speaker. “He called me two, three weeks ago — it was a very nice conversation. He was congratulating me — this was before we had the ultimate victory — but he was congratulating me and doing so well. I figured, routinely, he’d be behind it. And he, the other day, just did a big surprise because I’ve had so many endorsements.

Johnson reported from Omaha. Paul Kane, Robert Costa, David Nakamura and Carol Morello in Washington contributed to this report.