When Willkie Farr & Gallagher disclosed an ex-partner’s communications with opposing counsel in litigation over credit card “swipe” fees charged by MasterCard Inc. and Visa Inc., merchants opposed to a multibillion-dollar settlement in the antitrust case pounced on the revelation in an effort to undercut the deal.

Now, if some of those merchants have their way, a plaintiffs lawyer who purportedly shared confidential documents with ex-Willkie partner Keila Ravelo could lose $32 million in fees that he was already awarded in the case.