Stewart hit a two-run homer off of Lannan in the seventh, and the Rockies scored six runs in the eighth inning to rally past the Washington Nationals 8-2 on Tuesday night.
Lannan had baffled the Rockies for most of the game and the Nationals led 2-0 through six. But with two outs in the seventh, Clint Barmes hit a bloop single to right and Stewart hit a 1-0 pitch into the right-field seats for his first career pinch-hit homer.
"You talk about the game flipping on a pitch," Rockies manager Clint Hurdle said. "Lannan pitched as good a ballgame as I've seen here through seven innings -- except for one pitch. That enabled us to capture some momentum and take that forward."
Lannan allowed five hits and struck out eight in seven innings, but Stewart's fifth home run of the season spoiled his outing.
"I wish I could take the pitch back," he said. "It was a mistake."
The Nationals' four-game winning streak was snapped. The Rockies remained eight games behind Arizona in the NL West.
It was another hard-luck game for Lannan, who is 6-11 with a 3.55 ERA this season.
"He's given us a lot of quality starts without much to show for it," manager Manny Acta said. "He hasn't been the beneficiary of a lot of run support."
Stewart's home run gave the Rockies new life, and the Nationals' bullpen helped them get the victory. Colorado's six runs in the eighth came on just three hits. Three Washington relievers combined for four walks and two hit batters in the inning.
"You always see something new and I don't remember the last time I saw four walks and two hit by pitches in an inning," Acta said.
Luis Ayala (1-7) relieved Lannan to start the eighth and retired the first batter, but he loaded the bases and was relieved by Charlie Manning. Brad Hawpe hit his first pitch to center to score two, and then Manning walked Troy Tulowitzki and pinch hitter Seth Smith to make it 5-2.
"I don't know it was. I can't explain it," Manning said. "It was a night where everything was going wrong. Every mechanical thing I could do I was doing it."
Saul Rivera relieved Manning and hit Willy Taveras to score a run and walked Jeff Baker to drive in another. Matt Holliday, who began the rally with a one-out double, drove in a run on an infield single.
The Rockies sent 12 batters to the plate in the eighth, an inning Hurdle said was one of the stranger ones since he joined the Rockies organization.
"If we would go through a video tape I'm sure we'd fine one," he said. "It was a strange inning."
The Nationals took a 1-0 lead on De La Rosa in the second inning. Austin Kearns led off with a single, moved to third on Lastings Milledge's infield single and a throwing error by Garrett Atkins and scored on a sacrifice fly by Jesus Flores.
Washington took a 2-0 lead in the fifth when Ryan Langerhans scored on a wild pitch.
The Nationals had a chance to expand the lead in the seventh when they had runners on the corners and one out, but Emilio Bonifacio grounded into a double play.
An MRI on Nationals shortstop Alberto Gonzalez's right leg revealed a contusion of the hamstring. He is day to day. ... The Rockies will activate LHP Jeff Francis (left shoulder) from the 15-day disabled list on Wednesday. Francis will start Wednesday night's game against the Nationals. ... Washington OF Elijah Dukes (right knee surgery) took batting practice and jogged in the outfield before Tuesday's game. No timetable has been set for his return.