Paul DeJong handed the Nationals their second straight walk-off loss, capping a back-and-forth finish with a game-ending solo homer in the ninth inning of the St. Cardinals' 7-6 victory Monday night.
DeJong took Koda Glover (0-1) deep leading off the ninth on a 3-1 pitch. A night earlier, Ryan Madson allowed a game-ending ninth-inning grand slam to the Chicago Cubs' David Bote in a 4-3 defeat.
Washington's bullpen has blown saves in three of its past four games. All-Star closer Sean Doolittle has been on the disabled list since early July, and top setup man Kelvin Herrera went to the DL with right rotator cuff impingement last week.
The Cardinals recorded their 10th walkoff of the season and DeJong became the sixth different player to end a game in grand fashion.
The Cardinals have won six in a row and moved to nine games over .500 for the first time this season.
DeJong's 380-foot drive ended a wild final two innings.
Matt Carpenter and Jedd Gyorko homered in the eighth inning to put St. Louis up 6-4. Gyorko started the rally with a leadoff drive, and Carpenter followed with a three-run homer off Sammy Solis.
The Nationals tied it at 6 in the top of the ninth on RBI singles by Daniel Murphy and Matt Wieters off closer Bud Norris. Dakota Hudson (3-0) relieved Norris and stranded two baserunners by retiring Wilmer Difo and Adam Eaton.
Juan Soto and Bryce Harper homered for the Nationals, who have lost five of seven.
Gyorko sparked St. Louis' big eighth inning with his homer off Justin Miller. Kolten Wong and Patrick Wisdom then singled to set up Carpenter's 33rd homer. Carpenter has homered in seven of his past 10 games. He extended his major-league leading on-base streak to 31 games with a first-inning bunt single. He has 17 homers during that string.
Harper won a 10-pitch battle with starter Miles Mikolas by drilling his 29th homer leading off the fourth to lead 2-1.
Ryan Zimmerman added a run-scoring double in the second for the Nationals.
Jose Martinez had four hits for the Cardinals.
Mikolas gave up four runs on four hits over seven innings. He struck out four and walked one.
Tommy Milone started for Washington and gave up two runs on 10 hits over 4 1/3 innings.
LHP Gio Gonzalez (7-8, 3.39) will face RHP John Gant (4-4, 3.89) in the second of the four-game series.