The Cardinals now trail the Phillies 2-1, and face elimination Wednesday night at Busch Stadium.
Garcia, who went scoreless inning for scoreless inning with Phillies starter Cole Hamels through seven frames, gave up a 3-run homer to Ben Francisco with two outs in the seventh.
"He was outstanding," said Cardinals Manager Tony La Russa. "He made one mistake and the guy hit it out of the park. Usually that doesn't happen. He rose to the occasion. He'll be proud of himself forever for what he did tonight."
Garcia threw seven innings, giving up the three runs, and striking out three in his first post-season appearance. The lefty went 1-2-3 in three of his first four innings.
Francisco, who had a starting job out of spring training, but struggled this season, was pinch-hitting for Hamels, who threw six scoreless innings and struck out eight. The Cardinals stranded 14 runners.
"Every pitch mattered," said Hamels. "Every inning mattered. They had a great pitcher on the opposing team. so I knew that I couldn't let it get out of hand."
The Cardinals responded to Francisco's blast with a run in the bottom of the seventh when David Freese singled home a run off Vance Worley.
In the bottom of the eighth they loaded the bases, but Ryan Madson got Allen Craig to ground in to a 4-6-3 double play to end the inning.
The Cardinals scored again in the ninth off Madson, when Albert Pujols led off with a double, and was driven in by Yadier Molina.
But Madson shut the door, getting Ryan Theriot to end the game with a grounder to second.
Edwin Jackson will pitch for the Cardinals on Wednesday against Phillies starter Roy Oswalt.
A Phillies win puts them in the NLCS for the third time in four years.
A Cardinals win would send the series to a decisive Game 5 in Philadelphia on Friday.
"The heart and guts that this club has demonstrated in rallying like they did is just off the charts," said La Russa. "And there's no doubt in my mind that you will see it again tomorrow."
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