There's just one problem: Trump has spent the last year railing against his party's powerful donors.

After Trump abruptly wrapped up the GOP nomination last week, many major GOP fundraisers are playing the wait-and-see game as they mull over whether to help the New York billionaire. Deep-pocketed financiers, who have collectively poured hundreds of millions of dollars into efforts to elect past GOP nominees, say they are monitoring Trump's tone -- and waiting for his phone call.

Fred Malek, finance chair of the Republican Governors Association and a major GOP fundraiser, said the donors who are choosing to sit on the sidelines are looking for a more conciliatory Trump. They are seeking reassurance, he said, that the presumptive nominee has put the constant party-bashing from the primary season behind him.

"He seems to be taking the position that -- 'Hey, he's the nominee so you've got to get behind him.' Well, it doesn't work that way," said Malek, who noted he has not yet heard of Trump making overtures to big donors. "He's going to have to figure out that he's got to be more inclusive and bring people together, and I think he will."

The biggest clue as to whether Trump will change his tune will come Thursday, when he meets House Speaker Paul Ryan. The highly anticipated gathering comes one week after Ryan made the bombshell announcement to CNN's Jake Tapper that he was simply not ready to endorse Trump.

On Tuesday, Trump signaled that he was open to thawing the tensions: "I look very much forward to meeting w/Paul Ryan & the GOP Party Leadership on Thurs in DC," he tweeted. "Together, we will beat the Dems at all levels!"

I look very much forward to meeting w/Paul Ryan & the GOP Party Leadership on Thurs in DC. Together, we will beat the Dems at all levels! — Donald J. Trump (@realDonaldTrump) May 10, 2016

Even as she continues to battle Bernie Sanders in the Democratic primary, Clinton already has a significant leg up in general election fundraising. The former secretary of state and her husband, former President Bill Clinton, have access to an expansive network of donors and Democratic contacts dating back decades. Her campaign, the Democratic National Committee and pro-Clinton super PACs are expected to tap into that grid of supporters and raise hundreds of millions of dollars.

Deal with the RNC

The Trump campaign and the Republican National Committee are hashing out an agreement that will allow them to raise money jointly -- which also allows them to collect larger checks from donors. While that's standard procedure for the party and its presidential nominee, it has drawn outsized attention this cycle because Trump made the unorthodox move of self-funding much of his primary campaign.

The Trump campaign is expected to ink a deal with the RNC that will direct $2,700 of a donor's contribution to the candidate — the maximum individual contribution allowed to a campaign -- while the remainder is funneled to the national and state parties.

Wary of Trump being labeled a flip-flopper on the fundraising issue, the campaign is aiming to brand finance events as "victory" events that benefit the party, rather than fundraisers that boost Trump's presidential campaign.

To start tapping those resources, Trump himself will have to be active in efforts to win over donors he has isolated throughout a brutally negative primary season.

Trump's finance team already includes Anthony Scaramucci, a New York investor who has raised money for several other Republican presidential campaigns, and Silicon Valley venture capitalist Peter Thiel, according to a campaign adviser.

Major GOP donor and casino magnate Sheldon Adelson also said recently that he would support Trump . But it's not clear whether Adelson, who supported Marco Rubio's failed campaign, will be willing to cut multimillion-dollar checks for Trump the way he has for GOP candidates in the past.

Meanwhile, Charles and David Koch appear unlikely to support Trump's general election efforts. Charles Koch, in particular, was openly critical of Trump's candidacy during the primary. One source familiar with the deliberations said while the Koch network is fully committed to investing in Senate races, no final decision has been made about the presidential campaign and that there have been no discussions so far between between the two camps.

Other major donors like New York Jets owner Woody Johnson has stayed quiet, while billionaire investor Paul Singer declared that he could not support either Trump or Clinton.

Former Minnesota Gov. Tim Pawlenty said even after the tumultuous primary season, most Republicans will eventually come around to supporting Trump. But wooing the donor class will be a tricky task for Trump, he said.

"The divide in that group may be a little more deep," said Pawlenty, who heads the Financial Services Roundtable. "They're either hardcore conservatives and some of them may be concerned about Trump's conservative credentials. The other end of the donor continuum, I have what I call conservative business types who are somewhat concerned about his intemperate nature. So he's got probably more work to do."

Trump's money team

As Trump ramps up his efforts to build a finance organization, he's relying on a mix of relative newcomers and traditional GOP donors to help build his war chest.

Trump tapped Steven Mnuchin -- an investment executive who is little known in GOP fundraising circles and has donated to Democrats -- as his campaign finance chairman.

But he is taking a more traditional approach in his joint fundraising efforts with the RNC. Lew Eisenberg, the current finance chairman for the RNC, will serve as the national finance head for the joint effort between the Trump campaign and the RNC, according to sources familiar with the plan. Trump has also hired Eli Miller, deputy finance director for Marco Rubio's 2016 campaign, to serve as chief operating officer of his finance team. Miller's hire was first reported by The New York Times and confirmed by CNN.

The billionaire businessman also plans to tap his "massive rolodex" of personal contacts -- from business executives to members of his golf clubs to personal friends who have not traditionally been involved in campaigns -- to help fund the party's general election efforts, a campaign adviser said.

And Great America PAC, the main pro-Trump super PAC, is preparing to release a list of major GOP donors that have committed to writing checks to the group, according to PAC strategist Eric Beach. Although Trump disavowed super PACs during the primary, Beach says based on Trump's public remarks about general election fundraising, the group now has the "green light' to forge ahead with an eye on November. The group's goal is to raise $15 million through the end of the Republican convention, he said.

While previous GOP nominees have spent years curating lists of financial backers, Trump will have to build out his fundraising apparatus in just a few short weeks. Trump has said he wants to raise $1 billion to take on Clinton, but even Trump's supporters acknowledge that may be an unrealistic goal for a campaign with only a barebones infrastructure in place.

Corey Lewandowski, Trump's campaign manager, shrugged off concerns that Trump might be outraised in the general election.

"Donald Trump is going to have the ability to raise a lot of money for the party," Lewandowksi said, but "the bottom line number is not going to tell the whole story."

Lewandowski pointed to the campaign's modest campaign spending in the primaries, Trump's personal star power and his ability to weather a barrage of negative ads during the primary fights -- as proof that Trump can notch victories even when he's outspent.

"The American people are really smart and they will see through the false attack ads," Lewandowski said.

But some GOP donors say there's nothing Trump can do to win them over.

Mac Stipanovich, a Florida lobbyist who supported Jeb Bush's presidential campaign, recently penned an "open letter" to fellow Republicans pleading that they reject Trump. He told CNN that he will keep raising money for the Republican Party, but only for committees that have no connection to the presidential campaign.

"If the RNC is going to spend money to elect Donald Trump, then I won't give them money," he said.

Stipanovich said that ultimately, he doesn't believe Trump will enjoy the "uniform support usually enjoyed by a Republican nominee.

"I think that part of what Speaker Ryan has done is try to insulate his members and their campaigns from what may be a disastrous Trump affect down ballot," Stipanovich said. "He basically signaled: I'm not going to pressure you to get on board the Trump train."