Jordan Zimmerman gave up an early 2-1 lead, and the Nats dropped a 6-3 decision to the Chicago Cubs on Sunday.
Zimmerman (5-3) gave up four runs on 10 hits in five innings.
Ian Desmond hit a two-run homer in the second, his fifth of the season, that gave Washington a 2-1 lead.
A Jonathan Herrera RBI single tied the game at 2-2 in the fourth. The Cubs took the lead in the fifth on a two-run single by Chris Coghlan, and after a Yunel Escobar RBI single in the bottom of the fifth brought the Nats to within 4-3, Chicago added two in the sixth on RBI singles by Coghlan and Starlin Castro.
Kyle Hendricks (2-2) got the win for the Cubs, giving up three runs on six hits in five innings of work.
The loss drops the Nationals (30-27) a half-game back of the New York Mets in the National League East.