Donald Trump’s last Republican foe, Ohio Gov. John Kasich, ended his quixotic presidential campaign Wednesday, cementing Trump’s remarkable triumph as his party’s presumptive nominee and launching him toward a likely fall battle with Hillary Clinton. Some Republican leaders began reluctantly rallying around Trump, but others agonized over their party’s future.

The billionaire businessman vowed to unite the splintered GOP, even as he was bitingly dismissive of members who have been critical of his campaign.

“Those people can go away and maybe come back in eight years after we served two terms,” he said on NBC’s “Today” Show. “Honestly, there are some people I really don’t want.”

Trump’s challenge in uniting Republicans was abundantly clear Wednesday. Senate Majority Leader Mitch McConnell issued a frosty statement saying he had committed to backing the GOP nominee and noting Trump’s “obligation” to bring the party together. And in a remarkable move, the last two Republicans to occupy the Oval Office — President George H.W. Bush and President George W. Bush — made clear they would not be helping Trump win the White House.

Clinton, in her first remarks since Trump’s new status was crystalized, said she was more than prepared to handle the kind of deeply personal attacks that helped defeat Trump’s Republican rivals.

“To me, this is the classic case of a blustering, bullying guy,” the all-but-certain Democratic nominee told CNN.

The long and chaotic Republican primary came to an abrupt end after Trump’s decisive victory Tuesday in Indiana. His win pushed Texas Sen. Ted Cruz, his closest rival, out of the race, with Kasich following on Wednesday.

“The people of our country changed me,” Kasich said during an emotional speech announcing the end of his campaign. “They changed me with the stories of their lives.”

Trump’s takeover of the GOP marks one of the most stunning political feats in modern political history. A first-time candidate, he eschewed traditional fundraising and relied more on his own star power than television advertising to draw attention. He flouted political decorum with controversial statements about women and minorities, leaving some Republicans convinced he won’t be able to cobble together the diverse coalition needed to win the general election.

“It’s his party between now and November, but I don’t think it’s going to be his party after November,” said Peter Wehner, a former adviser to President George W. Bush. Wehner is among the Republicans vowing to never vote for Trump, even if that means essentially handing Clinton the presidency.

Bob Vander Plaats, an influential evangelical leader who backed Cruz, withheld his support for Trump Wednesday, saying the real estate mogul needs to prove his conservative credentials with his vice presidential pick and more information about what kind of judges he would appoint. “It’s kind of a wait-and-see moment with Mr. Trump,” he said.

Vander Plaats and a handful of other Republicans are clinging to the hope that an alternative option might yet emerge. Operatives have floated former Texas Gov. Rick Perry, Nebraska Sen. Ben Sasse and former Oklahoma Sen. Tom Coburn as possible candidates for a third-party or independent bid. But that is a long shot at best, with filing deadlines for getting on state ballots fast approaching.

There was notable silence from House Speaker Paul Ryan, who has spoken critically of Trump. Aides to both Ryan and McConnell said the Republican leaders had not spoken with Trump since his Indiana victory.

A spokesman for George W. Bush said the former president “does not plan to participate in or comment on the presidential campaign.” And a spokesman for Bush’s father said simply, “At age 91, President Bush is retired from politics.”

Still, several GOP governors and senators said they would support Trump, according to a survey by The Associated Press.

“Our first and foremost goal is to elect a conservative, pro-business, strong on national defense, a man who will stand behind our freedoms and our rights, and that person is Donald Trump,” Oklahoma Gov. Mary Fallin said. “It is not Hillary Clinton.”

Clinton has yet to shake Vermont Sen. Bernie Sanders, the democratic socialist who has energized young people and liberals with his calls for sweeping government-run health care and education programs. Still, Clinton has 93 percent of the delegates she needs to clinch the Democratic nomination and now is focusing her attention on Trump.

“He is a loose cannon and loose cannons tend to misfire,” Clinton said Wednesday.

Both Clinton and Trump head into the general election with historically high unfavorable ratings. But Clinton is generally popular within her own party, particularly with women and minority voters who are crucial to winning general election battleground states like Florida, Colorado and Nevada.

Some Republicans fear Trump’s poor standing with those voters will not only cost the party the White House for a third straight term but the GOP’s Senate majority as well. Some Republican senators in tough races struggled Wednesday to position themselves in a party with Trump at the helm. One, New Hampshire Sen. Kelly Ayotte, said that while she would support him in the election, she would not endorse his candidacy.

Trump turned quickly to the general election, saying he’s planning to work with the Republican National Committee to fundraise, a departure from his largely self-funded primary campaign. “We’re going to try over $1 billion, which is what’s going to be necessary,” he told NBC. He also said he was starting to vet potential running mates.

He later said he could consider Kasich, predicting the Ohio governor would be helpful this fall in any role given his state’s significance in the general election.

Associated Press writers Bill Barrow, Lisa Lerer, Stephen Ohlemacher, Laurie Kellman and Julie Bykowicz contributed to this report from Washington.