Carlos Beltran homered from both sides of the plate for the ninth time in his career and drove in five runs, leading St. Louis over the Nationals 10-4 on Sunday.
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"We're one step closer, but there's still a long way to go," said Mike Matheny, who took over as manager from Tony La Russa after the Cardinals won last year's World Series. "We've got to keep playing and playing well. The guys came out pushing today against a very good team."
St. Louis (86-73) took a 7-0 lead by the third inning and holds a two-game lead over the Los Angeles Dodgers (84-75), who beat Colorado 7-1.
Washington (96-63), already assured of the capital's first postseason baseball since 1933, saw its magic number for clinching the NL East remain at one over second-place Atlanta (93-66).
"We've had rough outings before," manager Davey Johnson said. "We've bounced back. It's better to clinch it at home anyway."
Beltran hit a pair of two-run homers, connecting right-handed off Ross Detwiler (10-8) in the second and left-handed against Chien-Ming Wang in the fourth. Beltran has four multihomer games this season and 34 in his career. He improved to 32 home runs with 97 RBI in his first season with St. Louis.
"Sometimes it happens where you feel from both sides, but most of the time you feel good from one side," Beltran said. "But it's always good to have a day like today where you have a good at-bat on the left and the right side."
Beltran entered in a 1-for-14 slide that had dropped his batting average to .265.
"As ballplayers, you're going to go through good stretches and bad stretches," Beltran said. "I try not to focus on that. I try to focus on the work that I do, come to the ballpark and prepare myself."
Matheny knows Beltran has been working hard.
"It's great to see for him," Matheny said. "The guy cares so much and it sometimes isn't translated in such a workmanlike attitude. It's really been bother him that he hasn't been able to step up. Days like today remind us of good he is."
Lance Lynn (18-7) allowed four runs and six hits in 5 1/3 innings, improving to 5-0 with a 2.10 ERA in his last five appearances.
"We won. That's all that matters," Lynn said. "You try to win series right now and do everything you can to win every game possible."
Pete Kozma was 3 for 3 with three RBI, raising his average to .338.
Detwiler, a St. Louis area native making his first start in his hometown, allowed seven runs -- three earned -- four hits and five walks in 2 1/3 innings. He set season highs for fewest innings and most walks.
"I just didn't throw any strikes," Detwiler said, who threw 43 of 81 pitches in the strike zone. "You walk five people in two innings, you won't have much success doing that."
St. Louis took a 5-0 lead in the second after Yadier Molina and David Freese walked, and second baseman Danny Espinosa booted Daniel Descalso's grounder, loading the bases. Kozma hit a two-run double down the right-field line, with Descalso thrown out at the plate on Espinosa's relay from Jayson Werth. Jon Jay's one-out single made it 3-0, and Beltran homered.
Harper's homer, Ian Desmond's RBI double and Espinosa's two-run homer cut the deficit to 7-4 in the fourth, but Beltran connected off Wang in the bottom half, and Beltran hit an RBI single against Zach Duke.
"The offense did great today," Lynn said "I was able to do my job, except for the fourth inning."
- John Lannan (4-0) will face the Kyle Kendrick (10-12) as the Nationals open a three-game series at home against Philadelphia on Monday.
- The Cardinals' Jaime Garcia (6-7) will oppose visiting Cincinnati's Bronson Arroyo (12-9).
- David Freese returned to the Cardinals' starting lineup after missing five starts with a sprained ankle.
- Harper is two home runs shy of Tony Conigliaro's record among teenagers.