Jamey Carroll got the go-ahead hit, one of three straight infield singles in a five-run eighth inning that led the Rockies to a 10-5 victory.
"We couldn't make plays," Nationals manager Frank Robinson said. "We couldn't make routine plays."
Washington overcame a 5-3, eighth-inning deficit when Austin Kearns hit an RBI single and scored on Brian Schneider's double off Jeremy Affeldt.
Troy Tulowitzki singled off Ryan Wagner (3-3) leading off the bottom half and stole second, and Jeff Salazar was walked intentionally. After pinch-hitter Vinny Castilla flied out, Carroll bounced a ball off the glove of Wagner, and Tulowitzki came home from second.
"Once the ball went off Wagner's glove, I knew I was going to be safe at first," Carroll said. "Then, I look around and see that Troy was halfway to home, and I was surprised."
Wagner blamed himself for not getting Carroll's bouncer.
"I've got to make that play," he said. "I'm a sinkerball pitcher that looks to get plays like that and make them."
Todd Helton followed with a grounder that Nick Johnson stopped behind the first-base bag as Salazar scored, and Garrett Atkins singled to shortstop as Carroll came home to make it 8-5.
"We score to get runs to get back into games then give them back," Robinson said. "I'm tired of seeing these kinds of games."
Matt Holliday, who earlier hit a two-run homer, hit a run-scoring single to right that chased Wagner, Felix Rodriguez relieved and Atkins scored when George Lombard failed to come up with Brad Hawpe's line drive to right, getting charged with an error.
"Lombard didn't lose the ball in the lights," Robinson said. "The ball missed his glove."
Manny Corpas (1-1) retired the final batter of the eighth to get the win in a matchup of last-place teams that started 2 hours, 11 minutes late because of rain.
"It has been a tough stretch for us," Colorado manager Clint Hurdle said, "so for us to answer back to the two runs they scored with five, and make some things happen there, was very encouraging."
Salazar, making his major league debut, was 2-for-2 with a walk, RBI and two runs.
Colorado took a 2-0 lead on sacrifice flies by Atkins in the first and Jeff Salazar in the second. After Alfonso Soriano's RBI single in the third, the Rockies made it 4-1 on Holliday's homer in the bottom half
Ryan Church hit an RBI double in the fourth, but Carroll chased starter Jason Bergmann with a bloop run-scoring single that gave the Rockies a 5-2 lead in the bottom half.
Johnson hit his 20th homer of the season in the sixth, a drive off Aaron Cook.
- Kearns was out of the starting lineup for a second straight game with a sore groin.
- Nationals OF Alex Escobar has surgery to repair a torn labrum in his right shoulder.
- Cook threw out Schneider from his knees after making a diving stop of a grounder.