Mr. Trump’s campaign spent just $12.4 million in 2015, according to disclosures filed with the Federal Election Commission, millions less than any of his leading rivals for the Republican nomination. More than half of Mr. Trump’s total spending was covered by checks from his supporters, who have thronged to his stump speeches and bought millions of dollars’ worth of “Make America Great Again” hats and T-shirts.

About $2.7 million more was paid to at least seven companies Mr. Trump owns or to people who work for his real estate and branding empire, repaying them for services provided to his campaign. That total included more than $2 million for flights on his own planes and helicopter, a quarter of a million dollars to his Fifth Avenue office tower, and even $66,000 to Keith Schiller, his bodyguard and the head of security at the Trump Organization.

While the convoluted accounting is required by law — so that Mr. Trump’s companies do not make illegal corporate contributions directly to his campaign — it also means that Mr. Trump is in effect taking millions of dollars out of one pocket and depositing it into another.

What remains is a quintessentially Trumpian endeavor that blurs the line between campaigning and brand-building and complicates Mr. Trump’s claims that he is funding his own White House campaign. About three-quarters of Mr. Trump’s total campaign spending has either gone to reimburse his own businesses or has been covered by funds from grass-roots donors, according to an analysis by The New York Times of F.E.C. reports. Virtually all of the money Mr. Trump himself has put into the campaign was lent, rather than donated outright, meaning that he could potentially sell enough hats and T-shirts to pay himself back down the road.