After Ben Sheets sparkled in a win in the opening-game of the doubleheader, the Braves failed to protect a two-run lead in the night cap, losing 5-2 Saturday night.
Bryce Harper returned from an injury sustained in the opener to single and score as a pinch-hitter, Roger Bernadina had a tiebreaking hit in the seventh inning and John Lannan earned a win in his first start of the season for Washington.
"It's hard to sweep doubleheaders in the major leagues and we had an opportunity," Braves manager Fredi Gonzalez said. "You go on to (today) with a chance to win the series and that's what we look for."
Sheets (2-0) pitched six shutout innings, Brian McCann and Chipper Jones homered in a 4-0 win in the first game.
Outdueling Nationals starter Edwin Jackson (5-6), Sheets extended his scoreless innings streak to 12 in his return after missing last season because of Tommy John surgery on his right elbow.
The Braves scored twice off Lannan (1-0) with two outs in the first inning. Jones stroked a two-out, RBI double and Freddie Freeman drove in Jones with a single. Atlanta never scored again.
"We had Lannan early," Gonzalez said. "He wiggled out of one situation, with a man on third and one out in the second and the top of the order up. We couldn't get that run in and they just kept tacking on single digits. We couldn't do anything after that."
The loss snapped Atlanta's three-game win streak and left the Braves 2½ games behind Washington in the NL East.
Randall Delgado, called up from Triple-A Gwinnett to make the start, allowed two runs on six hits over six innings.
Cristhian Martinez (4-2) allowed three runs, two earned, in 1 2/3 innings in relief of Delgado.
Bernadina, who entered as a defensive replacement after Harper fouled a bunt off his left ankle in the first game, had five hits on the day.
In between games, Nationals manager Davey Johnson said the MRI on Harper showed no break. He kept the 19-year-old out of the starting lineup for Game 2, but Harper entered as a pinch-hitter in the eighth with the Nationals leading 3-2.
The first-time All-Star singled, stole second and scored on Danny Espinosa's single to right. Shortstop Paul Janish's fielding error allowed Espinosa to eventually score.
In his second start of the season and with light rain falling intermittently, Sheets threw 91 pitches, allowing five hits with six strikeouts and three walks.
"It's been a huge pickup for us," McCann said of adding the four-time All-Star to the rotation. "To come out here for his two starts and pitch the way he has. Hasn't given up a run. He's pounding the zone and the more you're around him the more you know why he's so successful. He's a competitor, he knows what he's doing."
The same can be said of McCann with the bat in his hands.
His solo homer in the second inning, the 16th of the season for the Braves catcher, was his seventh in 10 games.
Lannan, recalled from Triple-A Syracuse, pitched seven innings and did not allow a hit over his final four frames. Tyler Clippard pitched the ninth for his 16th save.
With Washington trailing 2-0 in the fifth, Espinosa raced home from third with two outs on a Delgado wild pitch.
Washington loaded the bases with no outs in the sixth on two singles and a hit batter. Ian Desmond's RBI groundout tied it at 2.
Tied at two in the seventh, Martinez allowed a leadoff Sandy Leon, who moved to second on a sacrifice bunt. One out later, Bernadina's grounder into right field scored Leon.
"We got off to the start we wanted to in the second game and just stopped, for whatever reason after the first or second inning," said Jones, who had a two-run, pinch-hit homer in the first game. We stopped swinging the bats and we didn't pitch real well the last four innings of the game."
NOTES: Jones had three RBIs and now has 1,601 for his career. ... Nationals OF Steve Lombardozzi and Freeman had their hitting streaks end at 13 and 11 games, respectively in the opener. ... Braves starter Jair Jurrjens faces Ross Detwiler in today's series finale.