WASHINGTON -- If the Colorado Rockies faced contenders like the Washington Nationals more often, they might also be in the playoff race.
Nolan Arenado went 4 for 4, including his 35th home run and a triple, and drove in three runs as the Rockies beat the Nationals 5-3 on Sunday.
Arenado and David Dahl hit back-to-back home runs in the third inning off Nationals rookie Lucas Giolito.
Colorado took two of three from Washington and won the season series 4-2.
Since August 12, the Rockies have won three of five series with the victories coming over the NL East-leading Nationals and the Chicago Cubs, owners of the best record in the majors. Yet those gains were offset by three game sweeps by Philadelphia and Milwaukee.
"It's weird the way we're playing," Arenado said. "When we face the good teams, the best teams, we take it to another notch. ... When play bad teams, it feels like we get kind of complacent. We have to change that. Every game counts now for us."
Bryce Harper, Trea Turner and Wilson Ramos went deep for Washington.
Giolito (0-1) allowed four runs and six hits in five innings. Washington has dropped six of eight.
Chad Bettis (11-7) allowed two runs, both on solo homers, and five hits with six strikeouts over seven innings in his longest start since April 10.
The right-hander induced Ben Revere into an inning-ending double play with two runners on in the fifth to preserve a 4-1 lead.
"He used his entire pitch mix well, but especially his changeup," Rockies manager Walt Weiss said of his starter. "The double play he got in the fifth was big. That was a game-changer."
Adam Ottavino recorded the final three outs for his second save after Harper homered off Boone Logan leading off the ninth.
The Nationals officially recalled Giolito from Triple-A Syracuse before the start of Sunday's game and sent infielder Wilmer Difo to Double-A Harrisburg. Washington's top prospect allowed three of the first four batters to reach as Arenado's RBI single put Colorado up 1-0.
"They definitely jumped on the fastball, and when I wasn't able to locate it, they kind of made me pay," Giolito said.
Turner immediately tied it in the bottom of the first with a leadoff homer.
In third, Arenado's liner cleared the fence in left for a two-run blast and Dahl followed with a drive for a 4-1 lead. All three runs in the third came with two outs.
"It's that one bad inning that does you in," Nationals manager Dusty Baker said of Giolito's outing. "That was the one bad inning."
Arenado's fifth-inning triple was initially scored a three-base error on Harper misjudged a fly ball hit to sunny right field.
The third baseman now has 32 RBI in August. In his last seven games, he is batting .552 (16 for 29) with five homers and 13 RBI. Arenado also two, four-hit games in that stretch.
Harper went 1 for 4 a day after he was ejected in the 10th inning of Washington's 9-4 loss for arguing a called third strike.
Washington reliever Koda Glover's wild pitch landed flush on the left shoulder of home plate umpire Mike Muchlinksi in the eighth. As Nationals catcher Ramos simultaneously searched for the ball and checked on the crumpled umpire, Daniel Descalso scored from second base for a 5-2 lead. Muchlinksi remained in the game.
Arenado's 32 RBI in August is one shy of the franchise record set by Andres Galarraga in 1996.
Rockies: RHP Tyler Chatwood (10-8, 3.75), on the disabled list with a mid-back strain since Aug. 16, will have a rehab start Monday for Double-A Hartford at Richmond. Chatwood is expected to go five innings or about 75 pitches.
Nationals: RHP Joe Ross (right shoulder inflammation) allowed one run on three hits in one inning Sunday in his first rehab appearance with Triple-A Syracuse.
Rockies: Colorado returns home to begin a three-game series against the Dodgers. RHP Jon Gray (8-6, 4.61 gets the nod for the Rockies.
Nationals: RHP Tanner Roark (13-7, 2.99 ERA) faces Phillies rookie Jake Thompson (1-3, 9.78) in the first of 22 straight games for Washington against NL East opponents.