Trump and his staff have told some individuals -- including potential donors -- that as soon as the billionaire businessman secures the nomination, he will pivot and begin fundraising for a general election fight, the source said.
In the event that he wins, Trump will have to quickly scale up his campaign -- particularly if he's competing nationwide against Hillary Clinton's behemoth political organization. In private conversations, Trump and his aides have acknowledged that the Republican Party will need to be well-funded for the general election, and that Trump will work with the party to ensure that's the case.
Their message is "basically we're going to be a traditional party candidate," as far as fundraising is concerned, the source said.
Trump has prided himself on running an efficient primary campaign, funded largely from his personal wealth. At campaign rallies, he regularly touts that he won't be beholden to special interests -- from banks to health care companies to insurance providers -- because he's footing most of the bill for his presidential bid.
"They have no control over Donald Trump. I don't want their money. I don't need their money," Trump told a South Carolina crowd last month, as he slammed Texas Sen. Ted Cruz as a candidate who would be controlled by big banks and oil companies because of his donors.
Speaking in Maine on Thursday, Trump repeated the attack, this time against Florida Sen. Marco Rubio.
"I tell people if you had like a lightweight Rubio as president, he's all controlled by the special interests, 100%," Trump said. "I'm self-funding, they're not paying me anything."
"Mr. Trump is focused on the primary election," said Trump campaign manager Corey Lewandowski. "He's self-funding his campaign. There's no discussion at all about the general election."
GOP rivals like Cruz and Rubio have far outspent Trump in the primary. As of late January, Trump's campaign had spent about $24 million. Trump has loaned about $17.5 million to his campaign from the time he entered the race, through the end of January.
"He's spent no money on his campaign so far," the source said of Trump's shoestring campaign budget. "That won't happen in a general election."
Trump has said his campaign accepts small donations and insists they're unsolicited, even though his campaign website is emblazoned with two prominent "Donate" buttons.
He often brags that big donors have offered millions of dollars and he has so far turned them down. But under this shift, Trump would begin taking in significant sums, and tapping donors for tens of thousands of dollars apiece.
Last week, after endorsing Trump, New Jersey Gov. Chris Christie foreshadowed the candidate's about-face on a call with his biggest donors, suggesting Trump would raise money in a general election.
Cooperation with RNC?
Whoever the nominee, the Republican National Committee would play a prominent role in providing ground troops and a data operation to support the GOP candidate.
Trump has had a frosty relationship with the Republican Party and recently accused them of "illegally" using his name to raise money.
Still, his campaign -- as well as rival campaigns -- have already begun conversations with GOP officials about the infrastructure the RNC will provide to support the eventual nominee. That includes resources, such as field staffers nationwide and an expansive data operation.
But an RNC official said they have not yet begun discussions with the Trump campaign, or any of the other GOP contenders, about the fundraising aspect. A spokesman for the RNC declined to comment on Trump's general election fundraising plans.
Traditionally, the nominee creates a joint fundraising account with the RNC, which allows the candidate to simultaneously raise money for their own campaign, the national party and select state parties.
In 2012, the RNC didn't begin a joint fundraising agreement with Mitt Romney until April. Even then, it invited the other candidates in the race to take part in joint fundraising agreements as well.