Members of both parties say a bipartisan deal is imminent that would expand the requirement for federal background checks for more gun purchases.
An agreement could boost President Barack Obama's prospects for curbing firearms violence, though his effort's ultimate fate in Congress remains unclear.
Democratic Sens. Joe Manchin of West Virginia and Chuck Schumer of New York said they were on the verge of an agreement. An aide to Republican Sen. Patrick Toomey of Pennsylvania said they appear close.
Several Senate aides and lobbyists said the compromise would extend background checks to transactions at gun shows and online. They and Toomey's aide declined to be identified to discuss private talks.
Manchin and Toomey were expected to announce their agreement on Wednesday.