NEW YORK (AP) — Donald Trump is taking his first steps toward raising the massive amounts of money needed for a national presidential race, aiming to broaden his primary insurgency into a full-fledged general election campaign and unite the fractured Republican Party behind him.

Trump is reaching out to party heavyweights, hoping to repair his sometimes-strained relationships with the Republican National Committee and big GOP donors whom he bashed repeatedly during the primaries.

But in a sign of the work he has to do, House Speaker Paul Ryan said Thursday he is not ready to support Trump as the nominee. Former President George W. Bush and 2012 GOP nominee Mitt Romney said they do not plan to attend the party's national convention in July.

Upbeat still, Trump said in a brief interview with The Associated Press that his message has made Republican "the hottest party around."

"There is tremendous growth no one has ever seen before," he said.

Trump's campaign is trying to convert that energy into dollars.

On Thursday, he named a finance chairman, Steven Mnuchin, a private investor with ties to New York and Hollywood. Mnuchin "brings unprecedented experience and expertise" to the fundraising operation, the campaign said.

And Trump is taking pains to reassure party leaders that he wants to help Republican Senate and House candidates, some of whom are openly worried that Trump at the top of the GOP ticket will be a drag on their own campaigns.

"In order to really govern, we need majorities in the House and Senate. We're going to work with the party to raise money for down-ballot races to be successful," said campaign manager Corey Lewandowski.

Earlier this week, Trump's final GOP foes, Ted Cruz and John Kasich, dropped out, clearing his path. Still, many party leaders, including Bush and Romney, are keeping Trump at arm's length.

Their reluctance to embrace him sends an unmistakable signal to their fundraising networks, which include most of the GOP's best-connected donors.

"You might have a lot of these donors sit on the sidelines," said Spencer Zwick, who led Romney's fundraising efforts and now serves as Ryan's national finance chairman.

Trump, a billionaire who paid for most of his primary campaign by himself, acknowledges he would have to sell some of his holdings to muster the hundreds of millions of dollars for a general election bid, something he says he doesn't necessarily want to do.

He said Thursday he would be "putting up substantial money toward the general election," following the $36 million in loans he previously made.

Yet he's also beginning to take a more traditional route for his likely battle with Hillary Clinton, a Democratic fundraising powerhouse.

Mnuchin, who has never led a major political fundraising team, faces a gargantuan task.

Many major GOP donors have never heard of him — or even know how to pronounce his name (muh-NOO-chihn). Like his new boss Trump, Mnuchin has a record of giving both to Republicans and Democrats — including Clinton during her 2008 presidential run.

Anthony Scaramucci, a New York investor and Republican donor, said that while Mnuchin is an "unconventional choice," he is "uber talented and will exceed all expectations."

He said Mnuchin, a friend with homes in New York and California, has Wall Street and Hollywood ties and "is a great team-builder." Scaramucci, who earlier raised money for failed GOP candidates Scott Walker and Jeb Bush, said he plans to be part of that team.

Trump also hopes to tap into the RNC's existing fundraising network, but faces hurdles.

"High-dollar donors need to be convinced that Trump is going to be a serious candidate and won't embarrass them," said Charlie Spies, a veteran Republican operative with deep ties to party fundraisers.

This is all new to Trump. Through the end of March, he had raised $12 million, mostly from fans who clicked the "donate" button on his website or bought wares such as the ubiquitous red ballcap emblazoned with his slogan, "Make America Great Again," campaign finance documents show.

That contrasts with Clinton, who has raised some $187 million so far and began her general election fundraising effort back in November that can solicit huge checks for her campaign, the Democratic National Committee and state parties.

Clinton is spending the better part of this month raising money, including stops in New York, Michigan, California and Texas. Trump, as of Wednesday night, had not a single fundraising event on the books.

Trump's allies are just beginning to organize.

Ed Rollins, who ran Ronald Reagan's 1984 campaign, joined the pro-Trump Great America PAC as a senior strategist. "We're going to figure out what the Trump campaign needs and can't do by itself," Rollins said on a super PAC conference call Wednesday.

Trump's lean campaign team will also likely depend heavily on the Republican National Committee as he shifts to the 50-state task ahead. Yet the RNC is also counting on him to reciprocate by helping to fund its nationwide data operation and staffing expansion.

RNC members note that Trump has attracted many new voters to the party, who would likely be willing to give money. It's unclear, however, whether he can attract enough elite donors capable of writing checks upward of $350,000, Spies said.

Trump will also name a transition team and a vice presidential search committee. He says he has not truly begun the VP vetting process but will favor a Washington veteran. Trump told The AP that he will announce his choice at the convention.

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Bykowicz reported from Washington. Terry Spencer in West Palm Beach, Florida, and Steve Peoples in Washington contributed reporting.