The game's only goal gave Eastern Conference-leading United (9-4-3) its fourth straight victory.
D.C. had to weather a strong assault by the Union on the first half, when they put nine shots on goal, forcing three saves by goaltender Bill Hamid.
Rookie Antoine Hoppenot took a through ball from Freddy Adu in the 62nd minute but banged it off the post for Philadelphia (2-8-2), which lost in its first game under new Coach John Hackworth.
The Union remained winless in six games.
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