It was a sobering loss for the Rockies, who fell to eight games back of first place after Arizona beat Pittsburgh on Monday.
Everything looked great early for the Rockies.
Cook (14-7) effortlessly retired the side in the top of the first. In the bottom half of the frame, Colorado plated three runs after Matt Holliday hit an RBI double and Brad Hawpe drove in two more runs with a sharp single.
But Jesus Flores hit a towering solo home run to left in the second, and Cook's titanium armor finally cracked.
He gave up a single to pitcher Tim Redding to open the third, and Emilio Bonifacio followed with a single that advanced Redding to second. Then Cook, who has played Gold Glove-caliber defense this season, made a crucial error when he fielded Willie Harris' bunt and tried to get the forceout at third rather than throw to first.
Cook's throw sailed wide of Ian Stewart's target and rolled all the way to left field, allowing two runners to score. Troy Tulowitzki made a throwing error to first later in the inning, which allowed another run to score. When the inning finally ended, the Nats had scored four runs -- only one of them earned.
"I wasn't throwing the ball well, but if I don't throw the ball down the left-field line right there, we get the guy out at third and it's a totally different ballgame," Cook said. "We probably either have a chance to take the lead, or we're still playing right now."
Last year, when the Rockies stormed into the World Series, they set a Major League record with a .989 fielding percentage. Colorado came into Monday's game ranked 23rd in the Majors with 61 errors.
"That's something we pride ourselves on not doing," Hurdle said. "We did it tonight, and that opened the gates."
Nationals pitcher Tim Redding (8-6) settled down after the first to pick up the win. The only other run he gave up came in the sixth, when Cory Sullivan hit into a fielder's choice to score Garrett Atkins.
"He's had success for a reason," Hurdle said. "He kept the ball out of the middle of the plate with a lot better regularity after that inning."
Cook exited the game after giving up seven runs (four earned) on 11 hits in 5 2/3 innings. It was his third-shortest outing of the year, and the seven runs tied a season high.
"It's the first time -- with any team I've been associated with -- that we've scored five runs off Aaron, because he is so tough," Nationals manager Manny Acta said. "We caught a couple of breaks with the errors, but we did have some good at-bats against him, too."
Before the game, not even a fortune teller would have predicted Cook to slip up against the lowly Nats. Cook, an All-Star this season, was riding a three-game winning streak. He was also facing a team with the worst record in the league that was dead last in the Majors in runs scored.
But that, as Hurdle would quickly remind you, is what makes this game so fascinating -- and so frustrating.
"You gotta go play," he said. "That's the beauty of the game. You don't like it, but when you live it, you know it's always there."