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WASHINGTON -- The Washington Nationals are closing in on their first postseason berth and they're not easing up -- even against one of baseball's worst teams.

For the second consecutive game, Washington tied a franchise record with six home runs Wednesday night in a 9-1 rout of the Chicago Cubs.

Bryce Harper hit two homers and Gio Gonzalez won his 18th game. Adam LaRoche kept up his tear at the plate with three hits for the Nationals, who have won four straight and own the best record in the majors at 84-52. They maintained a 7 1/2-game lead over Atlanta in the NL East.

"Good teams take nothing for granted," Washington manager Davey Johnson said. "Guys down at the bottom, sometimes they have more energy and more to prove than guys at the top. We've been down there enough. We know what it's like, we're not letting up."

LaRoche has hit three of those 12 long balls, and his streak of reaching base in nine consecutive plate appearances was snapped by a seventh-inning strikeout.

LaRoche also has four singles and two walks in the past two games.

"It's just one of those stretches that nobody can explain. They don't happen too often," he said.

The 19-year-old Harper had his second multihomer game. Previously, only Mel Ott and Ken Griffey Jr. had accomplished that feat as teenagers.

"I don't think anybody's satisfied where we're at right now," Harper said. "We don't want to let down to anybody. We don't want to play at that level. We want to play at our level."

Gonzalez (18-7), who shut out St. Louis on five hits in his last start, allowed only three singles in seven scoreless innings to tie Mets knuckleballer R.A. Dickey for most wins in the majors. Dickey beat the Cardinals 6-2 earlier in the day.

Gonzalez struck out nine and walked none. He has yielded eight hits and no runs in his last 16 innings.

He says being a 20-game winner isn't on his mind.

"We're playing such great baseball, it's selfish of me to think of myself," Gonzalez said. "Individual stuff is nice, but all said and done it's a team thing."

Craig Stammen worked the final two innings and allowed a ninth-inning homer to Anthony Rizzo, his 11th.

Gonzalez did not give up a hit until Darwin Barney led off the sixth with a single past third baseman Ryan Zimmerman.

Starlin Castro singled off second baseman Danny Espinosa's glove with two outs in the seventh and Welington Castro followed with a single to left. Gonzalez finished the inning by striking out Josh Vitters.

Manager Dale Sveum, whose Cubs have lost five straight, was ejected in the fifth by plate umpire Larry Vanover for arguing balls and strikes.

"I just didn't appreciate him eyeballing our dugout for absolutely no reason at all," Sveum said.

"I just don't think that's right when you're looking in our dugout for no apparent reason at all. Nothing at all to warrant that at all."

The Nationals scored in the first on LaRoche's RBI single.

Washington equaled a club mark by hitting three home runs in the third off Chris Volstad (2-10). Roger Bernadina led off the inning with his fourth of the season. Harper followed with his 16th.

Zimmerman singled and LaRoche hit his 28th home run -- his fourth in three games.

Harper hit his 17th with one out in the sixth off Michael Bowden.

Ian Desmond and Espinosa hit back-to-back home runs to start the seventh off Blake Parker.

Volstad allowed five runs and nine hits in five innings. After breaking a 24-start winless streak that began July 17, 2011, he had won his previous two starts.

He was wary of pitching against Washington, which scored at least eight runs in three of four games last weekend against St. Louis.

"They put up runs all over the place against them, too. They're a hot team. A good team and they're hot. It's a tough combo, but you've still got to go out there and make good pitches," Volstad said.

Game notes

Harper's 17 home runs are the third-most by a teenager. Only Tony Conigliaro with 24 and Ott with 18 hit more. ... Vitters is in an 0-for-25 skid. ... Chicago is 17-51 on the road. ... Washington LHP Sean Burnett will be out a few more days with a tender elbow, Johnson said. ... Nationals OF Jayson Werth, who had four hits Tuesday but is 3 for 33 against Volstad, was given the night off. ... Nationals RHP Jordan Zimmermann (9-8, 3.01 ERA) will face Chicago RHP Justin Germano (2-5, 6.30) on Thursday.