"Nats Clinch" flashed on the scoreboard as Washington ensured at least an NL wild-card spot behind Ross Detwiler's six strong innings and Ryan Zimmerman's RBI double.
A crowd of 30,359 stood and cheered in the ninth inning, then got even louder when Drew Storen struck out Hanley Ramirez to end it. Manager Davey Johnson saluted the fans as he left the field and the team wore caps acknowledging the playoff berth.
Washington's magic number to win the NL East was reduced to eight. The Nationals lead idle Atlanta by 5 1/2 games.
Washington was last in the post-season 79 years ago, when player-manager Joe Cronin and the Senators lost to the New York Giants in five games in the World Series.
The loss dropped the Dodgers three games behind St. Louis for the NL's second wild-card spot. Milwaukee moved ahead of Los Angeles with its win over Pittsburgh.
Detwiler (10-6) allowed just Mark Ellis' fourth-inning home run and two singles. Storen earned his third save.
Chris Capuano (11-11) got the loss.
REDS 5, CUBS 3
CHICAGO (AP) — The Reds became the first team in the majors to clinch a playoff spot this season, beating the Cubs while manager Dusty Baker remained in a hospital after being diagnosed with an irregular heartbeat.
Ace Johnny Cueto (18-9) and the NL Central leaders ensured themselves of at least a wild-card spot. Cincinnati cut its magic number to two for winning the division for the second time in three years.
The Reds said Baker would remain in a Chicago hospital for an additional day so doctors could monitor his progress. He is hoping to return to Cincinnati on Friday. Bench coach Chris Speier ran the team for a second straight game.
Alfredo Simon got four outs for his first save.
The Reds broke a scoreless tie by getting five straight singles off Manuel Corpas (0-2) in the seventh during a five-run rally capped by Henry Rodriguez's two-run double.
PHILLIES 16, METS 1
NEW YORK (AP) — Jimmy Rollins and Chase Utley sparked an eight-run first inning, Ryan Howard added a late grand slam and the Phillies routed the inept Mets before a few hundred fans at quiet Citi Field.
Rookie right-hander Tyler Cloyd (2-1) pitched eight innings of three-hit ball and the Phillies stayed four games behind St. Louis for the second NL wild card, with two other teams in their path.
Juan Pierre matched a career high with five hits, Utley added four and Philadelphia set season highs for runs and hits (21) — including 19 singles. The Phillies finished a three-game sweep and won for the 11th time in 14 games.
New York rookie starter Jeremy Hefner (2-7) did not record an out in the makeup game for Tuesday night's rainout. With the Mets setting marks for futility at home, it appeared fewer than 1,000 fans were in the stands for the first pitch.
CARDINALS 5, ASTROS 4
ST. LOUIS (AP) — Pinch-hitter Carlos Beltran snapped a sixth-inning tie with a two-run double and Allen Craig hit a three-run homer, powering the Cardinals to a season sweep at home against the Astros.
Beltran's double put St. Louis up 5-3, rewarding Jaime Garcia (5-7) for keeping it close and leaving the Cardinals 2 1/2 games up for the second NL wild card with 12 games remaining. They took care of business against the lowly Astros, who absorbed their 102nd loss while leading for a single inning in the three-game series.
The Cardinals were 6-0 at home against Houston, their first season series sweep in St. Louis since 1996.
Bud Norris (5-13) allowed five runs in 5 1-3 innings for Houston after getting sent back to the team hotel Wednesday night suffering from flu-like symptoms.
Jason Motte finished for his 38th save in 45 chances.
BREWERS 9, PIRATES 7
PITTSBURGH (AP) — Rickie Weeks hit a tying two-run triple and scored the go-ahead run on Aramis Ramirez's single, helping surging Milwaukee rally for the victory.
The Brewers have won five straight, eight of nine and 15 of 19 to climb into the playoff race. They remained 2 1/2 games back of St. Louis for the second NL wild-card slot.
Milwaukee trailed 7-4 before it rallied for four runs in the eighth inning.
Ramirez had three hits and three RBIs for the Brewers, who built an early 4-0 lead.
Pittsburgh star Andrew McCutchen hit a three-run drive for his 30th homer. Chris Resop (1-4) got the loss.
The Pirates (74-75) have dropped three straight and 11 of 13 to slip under .500 for the first time since late May. They are in danger of a 20th straight losing season, which would extend their major North American professional sports record.
Manny Parra (2-3) picked up the win and John Axford of Simcoe, Ont., worked the ninth for his 31st save in 39 chances.
GIANTS 9, ROCKIES 2
SAN FRANCISCO (AP) — Pablo Sandoval homered from both sides of the plate and the NL West-leading Giants reduced their magic number to clinch the division to four.
Buster Posey also connected to back another strong start by Barry Zito (13-8) as San Francisco completed a four-game series sweep.
Zito won his fifth straight decision and has done everything he can to earn himself a spot in the post-season rotation after being left off all three rounds for the 2010 World Series champions.
The second-place Dodgers were set for a night game at Washington, trailing by 9 1/2 games. San Francisco (87-63) moved a season-best 24 games above .500.
Colorado has lost six in a row. Jorge De La Rosa (0-1) was tagged for five earned runs, six in all, and seven hits in 3 2-3 innings.
PADRES 6, DIAMONDBACKS 5
PHOENIX (AP) — Jesus Guzman homered to help San Diego get the victory.
Clayton Richard (14-12) allowed four runs and eight hits in six innings to raise his career record against Arizona to 6-0 — including three wins in three starts at Chase Field over the past 10 weeks.
Anthony Bass, the last of five Padres pitchers in the ninth, struck out Aaron Hill on a checked swing for his first career save.
Adam Eaton hit his first career home run and Hill also went deep for the Diamondbacks, who had won three in a row. Justin Upton had three hits and scored twice.
Diamondbacks starter Tyler Skaggs (1-3) pitched in and out of trouble while allowing four runs and seven hits over five innings.