Trailing in the polls and facing a cash crunch last September, Mr. Trump’s political operation began opting online donors into automatic recurring contributions by prechecking a box on its digital donation forms to take a withdrawal every week. Donors would have to notice the box and uncheck it to opt out of the donation. A second prechecked box took out another donation, known as a “money bomb.”

The Trump team then obscured that fact by burying the fine print beneath multiple lines of bold and capitalized text, a New York Times investigation earlier this year found.

The maneuver spiked revenues in the short term — allowing Mr. Trump to spend money before the election — and then caused a cascade of fraud complaints to credit cards and demands for refunds from supporters. The refunded donations amounted to an unwitting interest-free loan from Mr. Trump’s supporters in the weeks when he most needed it.