Two widows of men killed in the Gulf of Mexico explosion that led to the largest offshore oil spill in U.S. history say they fear Congress is losing interest in passing a measure soon that would allow them to seek damages in court for the tragedy.

Under current law, families of anyone killed at sea—rather than on land—are banned from receiving damages for loss of care and comfort. Congress is considering a measure that would change the law for families of workers who died in the BP PLC explosion.

Two...