Donald Trump assumed control of the Republican Party on Wednesday as its presumptive presidential nominee after Ohio Gov. John Kasich exited the race, moving swiftly to consider vice-presidential prospects and plan for what is expected to be a costly and vicious six-month battle for the White House against Democrat Hillary Clinton.

Trump, who has proudly touted how he has self-funded his campaign, said he would begin actively seeking donations for his campaign and raise money for the national party, part of the arduous task of coalescing a party deeply divided over his toxic brand of politics.

Party leaders are scrambling to stave off a parade of prominent Republicans endorsing Clinton, but already there were notable defections. The two living Republican past presidents, George H.W. Bush and George W. Bush, have no plans to endorse Trump, according to their spokesmen.

In the swing state of Nevada, Gov. Brian Sandoval, a moderate Republican and rising Latino star, said he plans to vote for Trump despite their disagreements on some issues. But Sen. Dean Heller (R-Nev.) said that “I vehemently oppose our nominee” because he disparaged women, Hispanics and veterans — although Heller insisted he would not vote for Clinton.

Democrats rushed to exploit the moment. The Clinton campaign released a brutal video mash-up of Republican rivals condemning his character and fitness for office, while the former secretary of state called him “a loose cannon” and invited Republicans and independents seeking an alternative to Trump to join her.

Republican presidential candidate Donald Trump held a campaign event at Trump Tower in Manhattan, after sweeping the Indiana primary. (Victoria Walker/The Washington Post)

“Let’s get off the red or the blue team. Let’s get on the American team,” Clinton said on CNN.

In states coast to coast, meanwhile, Democrats tried to link embattled Republican senators and other officeholders to Trump in hopes that the shrapnel from his polarizing candidacy would impair Republicans down the ballot. Some Republicans tried to keep mum about Trump, and others gave puzzling statements that sought to walk a tightrope between embracing him and distancing themselves from him.

As some conservative commentators lit up social media with images of burning GOP registration cards, some party elders called for a healing process and sought to quiet talk of an independent protest candidacy.

“Life is a series of choices, and this choice looks like one between Donald Trump and Hillary Clinton,” said Haley Barbour, a former Mississippi governor and national party chairman. “Anybody who proposes a third party is saying, ‘Let’s make sure Clinton wins.’ ”

Senate Majority Leader Mitch McConnell (Ky.) stood with Trump. “As the presumptive nominee, he now has the opportunity and the obligation to unite our party around our goals,” McConnell said in a statement.

Trump said he was hardly fretting about whether leading Republicans, such as 2012 presidential nominee Mitt Romney, would eventually back him.

“I believe that the people are going to vote for the person,” Trump said in an interview. “They love their party, but until this year the party was going in the wrong direction. . . . We’ve made the party much bigger.”

1 of 10 Full Screen Autoplay Close Skip Ad × 10 celebrities who say they’ll leave the U.S. if Donald Trump is elected president View Photos From Jon Stewart to Cher, some said they’d move to Canada, but some preferred another planet. Caption From Jon Stewart to Cher, some said they’d move to Canada, but some preferred another planet. Jon Stewart After the Emmy Awards in September, Stewart was asked whether he’d consider returning to "The Daily Show" if Donald Trump is elected president in 2016. “I would consider getting in a rocket and going to another planet because clearly this planet has gone bonkers," he said. Chip Somodevilla/Getty Images Buy Photo Wait 1 second to continue.

Trump spent Wednesday holed up in his soaring New York skyscraper, plotting ways to repair his image and destroy the opponent he calls “Crooked Hillary.” He said he was shell-shocked by his sudden emergence as the Republican standard-bearer, having anticipated that his fight with Sen. Ted Cruz (Tex.) and Kasich would continue until June’s California nominating contest. Both left the race in the wake of Trump’s resounding primary win Tuesday in Indiana.

“Who would have thought that I’d be here and we’d be waiting for Hillary?” Trump said, referring to Clinton being locked in a primary fight with Sen. Bernie Sanders (Vt.). “I was all set to move to California for a few weeks.”

Kasich, a career politician whose sunny campaign failed to gain traction in a year dominated by anti-establishment anger, suspended his bid Wednesday in an emotional speech tinged with wistful anecdotes about town-hall meetings he called “absolutely magic.”

“The people of our country changed me,” Kasich said in Columbus, Ohio. “The spirit, the essence of America lies in the hearts and souls of us. You see, some missed this message. It wasn’t sexy. It wasn’t a great sound bite.”

With Kasich and Cruz out, Trump and his advisers began making decisions about the general election. Though he has repeatedly touted his ability to self-finance his campaign, Trump said that he would seek donations going forward, especially small-dollar contributions from grass-roots supporters.

Trump acknowledged that he would have to liquidate some of his real estate holdings to muster the hundreds of millions of dollars needed to self-fund a credible fall campaign. “I mean, do I want to sell a couple of buildings and self-fund? I don’t know that I want to do that necessarily,” Trump said on MSNBC.

So far, Trump has given or loaned his campaign more than $36 million and accepted an additional $12 million in donations.

Trump said in an interview with The Washington Post that he would enter a joint fundraising agreement with the Republican National Committee and has scheduled a meeting Thursday with advisers to review the deal and finalize his finance strategy.

The arrangement would require him to seek support from a donor class that he has repeatedly excoriated. Top GOP financiers conferred privately about backing Trump. Among those still weighing his 2016 plans: hedge fund billionaire Paul Singer, who supported an anti-Trump super PAC and has not come around to the real estate tycoon, according to a person familiar with his views.

“He’s starting in a hole,” GOP campaign finance attorney Charlie Spies said of Trump. “He’s attacked all these people and said they are corrupt lobbyists.”

Recent White House nominees began assembling their fundraising operations as much as two years before the general election.

“I don’t see any way they can raise the hard dollars to be competitive,” said Fred Malek, who served as Sen. John McCain’s national finance chairman in 2008. “So unless Trump is willing to put in a substantial amount of his own money, he is going to have a mammoth financial disadvantage.”

Trump’s efforts could be bolstered by an allied super PAC, the Great America PAC, which the Trump campaign disavowed during the primaries. But in recent days, the group added professional operatives and now plans to court major contributors with Trump’s apparent blessing. The super PAC’s leaders held a donor conference call — which included retired neurosurgeon Ben Carson, a trusted Trump ally whose participation was seen as a de facto blessing — to signal it was now the go-to super PAC for wealthy Trump friends.

Ed Rollins, who managed President Reagan’s 1984 reelection and has joined the super PAC as senior adviser, said on the call: “He’s going to need help. The Democrat world is going to raise extraordinary sums of money. They’re licking their chops.”

Rollins said in an interview that the group is considering activities beyond advertising, including field organizing in battleground states, research and polling. “There are big donors who have said to me in the last couple of days, ‘Listen, we don’t want to waste our money. We want to help Trump,’ ” he said.

As presumptive nominee, Trump will help shape the programming of the party’s July convention in Cleveland, and the convention and Trump staffs will begin working together.

The Trump campaign will quickly expand beyond what has been a relatively skeletal staff to do battle with Clinton’s sprawling operation. “Everything is a priority,” campaign manager Corey Lewandowski said.

Trump said in the Post interview that he is weighing potential running mates. He said he prioritizes someone with governing experience and with whom he has a good rapport, citing Barack Obama’s selection of Joe Biden as a model. He said he will hire a law firm in the coming days to oversee the vetting process and that Carson will be part of the selection team.

“In all fairness, when Obama chose Biden, it was an odd choice, and yet they have very good chemistry together and therefore it was a good choice for them,” Trump said. “So having good chemistry is very important.”

Trump said he has his eyes on Kasich, saying that during intermissions at debates the two gravitated toward each other. “I’ve always liked him and I’ve always gotten along with him,” Trump said. Is he on the short list? “Let’s put it this way,” Trump said, “he’s rising rapidly.”

Trump said he is eager to start receiving regular classified intelligence briefings from the U.S. government — a tradition for party nominees — and said he hopes to work with GOP congressional leaders to coordinate a cohesive policy agenda for the fall campaign.

“I’m very much a team player, and I look forward to working with Mitch McConnell and Paul Ryan and Kevin McCarthy and everybody,” Trump said. He said he aims for “total cooperation,” though noted their clear differences on trade.

But Trump dismissed the idea of toning down his rhetoric and vowed to stand by his earlier, controversial calls to temporarily ban Muslims from entering the United States and to deport millions of undocumented immigrants.

“I really feel that it’s important that I do what’s right as opposed to necessarily cater to what’s going to play to the voters,” Trump said. “Because nobody really knows what plays with the voters, and I’m an example of that. If I tested some of the things I say out in polling, I probably wouldn’t do very well.”

Matea Gold, Mike DeBonis and Anne Gearan contributed to this report.