Marlins starter Burke Badenhop gave up three runs on five hits in three innings, with five walks and four strikeouts.
Kevin Gregg (2-0) pitched the ninth for Florida, giving up two runs on one hit and one walk, but picking up the win thanks to Helms' game-winning RBI.
Shawn Hill started for Washington, going five innings and giving up four runs on eight hits, with six strikeouts and no walks.
Saul Rivera (1-1) took the loss for the Nationals who lost for the fourth time in their last five games.
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