The team was stymied all day by Washington's pitchers, who combined to record 11 strikeouts. Starter Tim Redding allowed just one run in 6 1/3 innings of work.
"We have a good offense, and we should be doing more," manager Tony La Russa said. "The games were there to win."
It was a repeat of Thursday's 11-inning game in Pittsburgh, and the two losses have come as the Cardinals are trying to catch up to the leaders in the National League Central.
One bright spot for the team is that pitcher Kip Wells was able to give the bullpen some much-needed rest. Wells threw 116 pitches in six innings of work, struggling at times but finishing with just two runs allowed.
The Cardinals tied the game in the eighth when Yadier Molina's single brought home Juan Encarnacion. After St. Louis held Washington scoreless in the bottom of that frame, Ryan Franklin was brought in to pitch the bottom of the ninth.
Franklin's first batter was pinch-hitter Tony Batista, who hit a broken-bat blooper that fell for a single.
"You just don't expect bloopers like that," Franklin said. "It hits you a little harder when you make your pitch."
Three batters later, Ryan Zimmerman sent home the winning run with a single, and the Cardinals fell to 50-56 on the season.
Wells received no decision for his six-inning outing, which was critical in providing a break to a bullpen that worked six innings on Thursday.
"He definitely pitched a winning game," La Russa said. "He pitched very well."
The lefty had gotten off to a rough start, though, loading the bases in the first. He then worked Ryan Church to a 3-2 count before delivering a called strike to end the inning.
Wells started the second inning by throwing seven consecutive balls before receiving a visit from pitching coach Dave Duncan.
"He basically reiterated the fact that I need to be efficient and give the team a boost as far as going deep into the game," Wells said. "Knowing the situation, I knew I was going to be out there as long as I was having good results."
Wells' pitch count of 116 was his second highest of the season. He threw 125 in five innings against Los Angeles on May 16.
Wells said that Friday's game was different because of the sweltering Washington heat. In addition, he hit a pair of singles and had to run the bases.
"It was just a taxing night, on lots of fronts," he said. "My arm didn't necessarily feel tired, but by the same token, I just kind of ran out of energy at the end."
His two singles, one of them an RBI hit, weren't the only offensive contributions from pitchers, as Redding crossed the plate to tie the game at 1 in the fifth inning.
Redding had the Cardinals flummoxed throughout his 6 1/3 innings of work, striking out a career-high eight batters.
When the Cardinals return to RFK Stadium on Saturday night, they will look for the right combination of pitching and hitting. They're now seven games behind Milwaukee, which has sole possession of first in the division.
Franklin vowed not to let a pair of agonizing finishes get the team down, saying that the Cardinals are just a couple of plays away from turning the corner.
"As far as I can see, we're right there," he said. "Both these games could have gone our way."
Michael Phillips is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.