Joe Thatcher took the mound and promptly uncorked a wild pitch, moving the runners up, and Bryce Harper followed with a sacrifice fly to tie it up.
Luke Gregerson came in to pitch and Zimmerman greeted him with a solid hit to right, plating Desmond with the decisive run.
Everth Cabrera then scorched a grounder to first that LaRoche speared with a dive and tossed to Soriano to end the game, giving the Nats closer his 24th save.
Harper finished with three RBI and LaRoche pounded a solo homer for Washington, which won a third straight.
Jesus Guzman smacked a three-run homer for the reeling Padres, who dropped an eighth consecutive game.