Payday lenders fought, and lost, a battle to block new federal rules curbing short-term loans that critics say can trap people in cycles of debt.

Now, with the restrictions scheduled to take effect next year, the lenders have moved their fight to the courtroom, and have gained a powerful ally: the Consumer Financial Protection Bureau, which wrote the rules the industry is seeking to overturn.

In a joint motion filed late last week in federal court in Austin, Tex., Mick Mulvaney, the bureau’s acting director, sided with two industry trade groups suing the agency. The bureau asked a judge to delay the rules until after the industry groups’ lawsuit is resolved, which may take years.

By then, the entire issue could be moot: Mr. Mulvaney has said he intends for the bureau to reconsider, and perhaps repeal, the rules. To do that, the agency has to follow a formal administrative process, which it has said it plans to begin by February.