"You can't throw the ball away," Bumgarner said. "I screwed it up for us."
The Giants still lead the best-of-five series, 2-1. But Tuesday's Game 4 at AT&T Park (6 p.m. PT on FOX Sports 1) is overflowing with urgency. Should the Nationals capture that game, the teams would return to Washington for the deciding contest Thursday.
"We want to win it tomorrow," Giants shortstop Brandon Crawford said after Monday's loss. "I'm sure nobody here wants to fly back to Washington."
Nationals starter Doug Fister blanked the Giants on four hits in seven innings, denying them their seventh consecutive triumph in a potential series-clinching game. Giants second baseman Joe Panik, who went 0-for-3 off Fister, praised the right-hander's repertoire.
"My first at-bat, he came in with a cutter," Panik said. "Second at-bat, he got me with a pitch away. Third at-bat, a changeup. There wasn't a set pattern."
San Francisco couldn't score until Crawford hoisted a ninth-inning sacrifice fly off Drew Storen that followed Pablo Sandoval's leadoff single and Hunter Pence's double.
Fister appeared vulnerable in the second inning, when the Giants had runners on first and second with one out. Bryce Harper then hauled in Crawford's drive to deep left field. Travis Ishikawa walked to load the bases before Fister struck out Bumgarner, the NL's top hitting pitcher.
"I think I may have swung at ball four," said Bumgarner, who slugged two grand slams this year.
That paled alongside the lapse Bumgarner committed in the seventh. Like Fister, Bumgarner remained infallible through most of the afternoon, blanking Washington on four hits through six innings. It was as if the teams agreed to resume Saturday's 18-inning standoff.
Ian Desmond's leadoff single to left field, which was struck with authority, provided the first sign of trouble for San Francisco in the seventh. The sense of foreboding grew as Bumgarner walked Harper, the lone free pass he issued all afternoon.
Bumgarner then pounced on Wilson Ramos' sacrifice-bunt attempt between the pitcher's mound and the first-base line and, urged by catcher Buster Posey, flung one of his 95 mph fastballs toward third base. Posey believed that Bumgarner could start a double play that would douse the Nationals' threat.
"I thought the way the ball jumped off the bat that he might have a shot," Posey said.
Bumgarner's throw scooted past Sandoval, who was knocked sprawling by Desmond and thus couldn't recover the ball. Ramos advanced to second base on the play and scored on Asdrubal Cabrera's single to left field.
The runs ended Bumgarner's postseason scoreless-innings streak at 22, second only to Christy Mathewson's 28-inning streak in franchise history. But the Giants didn't dwell on that. Bumgarner expressed annoyance because he believed that Ramos could have been retired at first base for a double play had he made a decent throw to third.
"I just threw it away," Bumgarner said, declining to cite excuses for his error. "I feel comfortable throwing to bases. It just got away from me."
Said Posey, "I made a mistake telling him to throw to third base. It happens."
Harper's ninth-inning homer off Jean Machi concluded Washington's scoring. That left the Giants to rely on Ryan Vogelsong, their leading winner (3-0) in the 2012 postseason.
"It's an honor to take the ball tomorrow," he said about his Tuesday assignment, "and I'm going to do the best I can for all the guys in [the Giants clubhouse] and all the fans in the stands."