Detwiler (3-5) gave up only three hits while striking out three and walking one batter. Detwiler also contributed offensively with an RBI single. Drew Storen worked a 1-2-3 ninth to record his 40th save of the season, and second of the day.
"I just focused on keeping the ball down," Detwiler said. "I felt I got stronger as the game went on."
Cliff Lee (16-8) was tagged with the loss after, giving up three runs -- two earned -- on 11 hits over seven frames while striking out nine without walking a batter.
"It was just one of those games where they got hits," Lee offered. "The ball Espinosa hit was hit really well. A 1-0 fastball out over the plate and he hit a home run there. Other than that, I felt like the other (hits) were not all that bad pitches."
Philadelphia, which has clinched the National League East and home field advantage throughout the playoffs, has struggled lately, losing the first game of the twin bill, 4-3, and losing four straight games for the first time since a four-game skid from May 31 to June 4.
Washington grabbed the lead in the second inning when Espinosa launched a two-seam fastball from Lee into the right-field bullpen to give Washington a 1-0 lead.
The Nats threatened again in the fourth when Michael Morse singled to right, Johnny Gomes reached thanks to an error by Chase Utley and Espinosa beat out an infield single to load the bases. Lee worked his way out of trouble by striking out Chris Marrero and getting Wilson Ramos to groundout to shortstop to keep the deficit at one.
Espinosa lined a single into right field, while Ramos ripped a single to left to put two runners on base with two outs in the sixth. Lee then left a fastball out over the plate to Detwiler and he lined it into center field, bringing home Espinosa for a 2-0 lead.
Washington was gifted another run in the seventh when Jayson Werth drove a double into the right-center field gap to start the frame and advanced to third on a single by Morse. Carlos Ruiz then threw down to third, trying to pick off Werth, but the throw hit Werth in the elbow and rolled in foul territory down the left field line, allowing Werth to trot home.
The Phils finally mounted a threat in the home seventh, albeit without a hit. Polanco drew a one-out walk and Utley was plunked by Detwiler two batters later. Philly was unable to break the goose egg, as John Mayberry Jr. lofted a fly ball to Gomes in right to end the inning.
Werth left the game in the seventh inning after getting hit with Ruiz's throw...It was Werth's 16th start of the season in center field...Tuesday's doubleheader is the Phillies third of the season and third against the Nationals since 2005...Desmond's home run gives him 20 for the season, which matched the franchise's single-season record for home runs by a rookie...The Phillies have been held to three runs or less in 12 of their last 13 games...Detwiler threw only 81 pitches in his 7 1/3 innnings of work.