The Biden administration on Friday brought new, sweeping accusations of fraud against three of the largest banks in the country — but President-elect Donald J. Trump’s administration will have the opportunity to drop the case as soon as next month.

The allegations involve Zelle, the money transfer app co-owned by seven of the nation’s largest banks that competes with Venmo, Cash App and other smartphone-based payment apps. As has been regularly reported, Zelle is a hotbed for scammers, and the lawsuit on Friday from the Consumer Financial Protection Bureau said that more than $800 million had been swindled from clients of Bank of America, JPMorgan Chase and Wells Fargo.

While fraud on Zelle can involve any number of complicated steps, it can be as simple as a grifter convincing an individual to send payment for an item, such as concert tickets, and then failing to deliver the promised good or service. Banks have long argued they aren’t responsible for reimbursing such losses to customers who voluntarily transfer money out of personal accounts, but the C.F.P.B. said they ought to be. In a 91-page complaint submitted in Arizona federal court, the agency laid out some of what it said were hundreds of thousands of instances of fraud enabled by the three banks.

“What they built became a gold mine for criminals — a system that made it easy for fraudsters to move money quickly while making it nearly impossible for customers to get their money back,” C.F.P.B. director Rohit Chopra said.

The lawsuit was the latest in what has become a late blitz of action from the C.F.P.B., one of the more hated agencies for many Republicans, who argue that it is reflexively anti-business. Earlier this month, the incoming chairman of the Senate Banking Committee, Senator Tim Scott of South Carolina, unsuccessfully called for the C.F.P.B. to pause its rule making until Mr. Trump takes office.

Another issue is the possibility of a government shutdown at midnight Friday. During past shutdowns, regulators and enforcement agencies have operated with minimal staff and been able to bring only emergency actions.

In a call with reporters, C.F.P.B. officials didn’t directly answer questions about their expectations for the new administration, as well as queries on why they sued only three banks when seven own Zelle. They said that more than 70 percent of payment activity occurs on the three charged banks.

A spokeswoman for Zelle said in a statement that the complaint was “legally and factually flawed,” and that its timing “appears to be driven by political factors unrelated to Zelle.” She said that the company made earlier efforts to discuss the matter with the C.F.P.B., without luck.

A JPMorgan Chase spokeswoman said the lawsuit amounted to regulators “making banks accountable for criminals.”

Bank of America and Wells Fargo declined to comment.

Matthew Goldstein contributed reporting.