The Nationals failed in their bid to win a season-best five games in a row.
Milwaukee climbed back to within a game of .500 and, despite being in fourth place in the National League Central, stayed within 4 1/2 games of first.
Washington starter Garrett Mock was still on the mound in the sixth, trying to protect a 5-3 lead. Mock, however, gave up a lead-off double to Corey Hart and a single to Mike Cameron that moved Hart to third.
Tyler Clippard (1-1) came on to relieve Mock and got J.J. Hardy to ground into a double play that brought home Hart from third.
With a chance to get out of the inning, Clippard walked Mike Rivera on a 3-2 pitch and then gave up McGehee's homer to left.
Manny Parra (5-8) survived six innings to get the win. He allowed five runs on nine hits while striking out seven.
Washington had been trying for its first five-game winning streak since last August.
Copyright 2009 by United Press International.