WASHINGTON -- Ian Stewart launched a three-run homer in the fourth inning, and the Rockies made multiple strong defensive plays in completing a three-game sweep of Washington, 7-2, at Nationals Park on Sunday afternoon in front of 31,467 fans.

The victory was the Rockies' ninth in their past 13 road games. Also, Rockies starter Aaron Cook (15-8), whose start was backed up three days because of back trouble, gave up two runs in five innings. Cook and the Reds' Edinson Volquez are tied for second in the National League in wins, behind the D-backs' Brandon Webb (18).

The Nationals have now lost a season-worst 10 straight games.

Stewart's eighth big league homer of a season split between Triple-A Colorado Springs and the Rockies came off Nationals starter Odalis Perez (5-9). The left-handed-hitting Stewart, the Rockies' first-round pick in the 2003 First-Year Player Draft, is on a 13-for-29 tear against left-handed pitching.

The Rockies had a one-run lead until the eighth, when Brad Hawpe homered to right off lefty Charlie Manning for his second home run of the series. Seth Smith added a three-run shot in the eighth off Jesus Colome.

Cook gave up seven hits, walked three and struck out four. He had help in the second inning with runners on second and third and one out. With the Nationals up, 1-0, second baseman Clint Barmes charged to field Pete Orr's chopper and threw home to retire Ryan Langerhans.

First baseman Garrett Atkins' diving stop in the fifth kept a one-run inning from becoming worse, and left fielder Matt Holliday made two impressive catches behind reliever Manuel Corpas in the sixth.

Thomas Harding is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.