Matt Leas, a spokesman for the C.F.P.B., said the agency has “a fair, transparent and thorough” process for making rules. “The comments received and evidence obtained are all taken into consideration before issuing a final rule,” he said. “The director is the ultimate decision maker and ensures that the decisions taken are justified publicly, as is required by law.”

Efforts to dismantle the payday regulation began with the arrival of Mick Mulvaney, the Trump administration budget chief, who was appointed the C.F.P.B.’s acting director in late 2017. Among his priorities was to delay, and eventually undo, the Obama-era payday lending restrictions, which were scheduled to take effect in summer 2019, according to two former senior bureau officials who discussed the issue with him.

But unwinding a federal rule is a long and complicated process. The bureau’s research team had already spent nearly five years gathering and analyzing data on payday lending for the original rule. To avoid having its reversal struck down in court — as dozens of Trump administration regulations have been — the bureau would need to demonstrate that new research or other data had called into question the original rule. That task fell to the agency’s office of research — a group of economists and other social scientists that included Mr. Lanning.

In 20 states, payday lending is effectively banned, but in the places where it remains legal, it has thrived: There are more payday loan storefronts in America than McDonald’s restaurants. Their customers — often working poor people who cannot always secure traditional credit — collectively borrow nearly $29 billion a year and pay nearly $5 billion in fees, according to research by Jefferies, an investment bank.

While short-term loans are intended as an emergency stopgap, many borrowers find themselves unable to repay their debts quickly, and borrow again. Half of all payday loans are part of a sequence that is extended at least nine times, piling up fees that can exceed the value of the original loan, according to research the consumer bureau published to support its original restrictions.