The Marlins, who came into the series in the thick of the playoff chase, lose another game to the Nats on Thursday, this time blowing a 6-0 lead in route a 12-8 loss.
Ryan Zimmerman finished a double short of the cycle, and Ronnie Belliard's tiebreaking single in the eighth inning helped Washington rally from a six-run deficit.
The Nationals broke an 8-all tie with four runs in the eighth, completing a series sweep and winning their season-best fifth straight game. The Marlins had won the first 10 games this season against the lowly Nationals.
Washington also scored six runs in the eighth inning for a 6-4 victory on Tuesday, and have come from behind for 20 of its major-league low 37 wins. They did it after Florida took early leads of 6-0 and 8-3, which the Marlins built on three hits in the first five innings by Hanley Ramirez and John Baker's two-run homer.
"They did a great job of swinging. They're playing really well right now," said starter Chris Volstad. "We've got to put this series behind us. That's really all you can do."