By turning the loans into donations, Trump's campaign cannot repay the candidate, even if a surplus arises. The campaign, however, can continue to reimburse Trump's companies, and those of his children, for campaign-related expenses, including travel.

And while individuals are limited to giving $2,700 per election to a campaign committee, a candidate can give unlimited sums to finance his or her own bid.

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Since Trump sent an email Tuesday asking supporters to contribute online, more than $6 million has come in, finance chairman Steve Mnuchin told CNBC. Separately, a high-dollar fundraiser held Tuesday night in New York in conjunction with the Republican National Committee brought in more than $6 million, according to people familiar with the figures.

What Donald Trump is doing on the campaign trail share Share View Photos View Photos Next Image U.S. Republican presidential nominee Donald Trump speaks at a campaign event at Trump Doral golf course in Miami, Florida, U.S. July 27, 2016. REUTERS/Carlo Allegri (Carlo Allegri/Reuters)

"After self-funding his primary election, Mr. Trump and the campaign have assembled an exceptional fundraising operation, which in recent days has been overwhelmed with contributions for the Republican Party," the campaign said in a statement Thursday.

But many stalwart party financiers have balked at writing him a check, noting that Trump bashed wealthy donors throughout the GOP primaries and touted that he was self-funding his campaign. Most of Trump's money to his bid came in the form of loans — $45.7 million through the end of May. That spurred worries in the donor class that he would simply use money they gave to pay himself back.