WASHINGTON — Two key senators have reached a bipartisan deal on expanding background checks to more gun buyers, a Senate aide and lobbyist said Wednesday, an agreement that could build support for President Barack Obama’s drive to curb firearms violence in the wake of the elementary school shootings in Connecticut.

Sens. Joe Manchin, D-W.Va., and Patrick Toomey, R-Pa., planned to announce their compromise later Wednesday morning. Subjecting more firearms purchases to federal background checks has been the chief goal of Pres. Obama and gun control supporters, who promote the system as a way to prevent criminals and other potentially dangerous people from getting the weapons.