Jason Motte (4-5) came in trying to protect a 3-2 lead for Jaime Garcia and retired his first two batters. But Andre Ethier kept the Dodgers alive with a single and pinch-runner Dee Gordon stole second before Cruz drove him in with a drive that center fielder Jon Jay barely missed on a diving attempt in front of the warning track.
Ronald Belisario (6-1) pitched 1 1-3 innings for the victory. Teammate Matt Kemp helped make it possible in the top of the ninth. He crashed into the center field fence trying to catch Yadier Molina's drive over his head, but recovered in time and kept him from stretching his hit into a triple with a brilliant somersaulting throw to Cruz.
On Friday night, Cruz helped beat the Cardinals 8-5 with a go-ahead three-run homer in the sixth against Mujica.
The Dodgers' third victory in 10 games tied them with the Cardinals for the second wild card spot with 16 remaining for both clubs. The Cardinals lost despite a homer and three RBIs by Allen Craig.
After this four-game series concludes on Sunday, the Cards will play their next nine games against the Astros and Cubs - who are a combined 83 games under .500 and 69 1-2 games out of first place in the NL Central. The defending World Series champions put themselves into this precarious position by losing 13 of their last 28 games.
Joe Blanton gave up three runs and four hits in 5 2-3 innings and struck out six. The right-hander is 1-5 with a 6.07 ERA in eight starts since joining the Dodgers in a trade with Philadelphia on Aug. 3.
A disputed call on a bang-bang play at first base enabled St. Louis to snap a 2-all tie in the sixth. Garcia drew a leadoff walk, ending a string of 13 consecutive batters retired by Blanton, and Jay followed with a single. One out later, Matt Holliday hit a potential double-play grounder to shortstop Hanley Ramirez and just beat the relay to first from second baseman Mark Ellis.
Dodgers manager Don Mattingly went out to argue with umpire Dana DeMuth, and shortly after he returned to the dugout, Craig lined a single to left on an 0-2 pitch to drive in Garcia and chase Blanton.
Craig, batting an NL-best .393 with runners in scoring position, missed the first month of the season on the disabled list while recovering from a fractured right knee cap and sat out another 14 games in May with a left hamstring strain.
Blanton, who has surrendered an NL-worst 29 home runs, gave up Craig's 21st of the season in the first inning after a two-out single by Matt Holliday.
The Dodgers got one of the runs back in the bottom half when Shane Victorino reached on a throwing error by Descalso, stole second and scored on a two-out single by Adrian Gonzalez. Mark Ellis tied it in the third with a leadoff homer into the lower seats in the left field corner on a full count.
Before the game, the Dodgers announced that reigning NL Cy Young Award winner Clayton Kershaw would not make his scheduled start in Sunday's series finale because of inflammation in his right hip. The 24-year-old left-hander, who will go to New York to see a hip specialist on Tuesday, may have to undergo arthroscopic surgery if the pain persists. It's another blow to a rotation that lost starters Chad Billingsley and Ted Lilly to season-ending shoulder injuries.
Kershaw, 12-9 with a 2.70 ERA and NL-leading 206 strikeouts, also was scratched from a scheduled start last Sunday in San Francisco before coming back with seven innings last Monday at Arizona and giving up an unearned run and three hits in a 1-0 loss.
NOTES: Cardinals RF Carlos Beltran did not start because of tightness in his lower back. ... Kemp, whose swing hasn't been the same since he crashed into the CF fence at Coors Field on consecutive days (Aug. 27-28), struck out three times in a game for the fifth time this season. He is 5 for 48 with a homer and two RBIs in his last 12 games since the Dodgers left Denver. ... Blanton has a career record of 6-13 with nine no-decisions when allowing a home run in the first inning.
Link to boxscore: Los Angeles 4, St. Louis 3