WASHINGTON -- The game was scoreless going into the eighth inning and Ryan Dempster wanted to stay in. Interim Cubs manager Mike Quade had other ideas.
The Cubs completed a three-game sweep that kept Quade unbeaten in three games since Lou Piniella retired. Quade is the first Cubs manager to win his first three games since Jim Riggleman, now guiding the Nationals, started 4-0 in 1995.
"He's undefeated," Dempster said. "He's really doing a great job. It's good for him he's getting a great opportunity to go out there and manage. I've always said we never quit. Sometimes we just haven't played as well as we want to. We're not going to quit for him and, more importantly, we're not going to quit for the entire team."
Dempster allowed two hits and struck out eight.
"He wanted no part of coming out," Quade said. "He wasn't happy about that and that's OK, I don't want guys happy. But I think you've got to try and win the game."
Colvin drew a leadoff walk in the eighth, stole second and scored on Starlin Castro's double. Aramis Ramirez hit his 20th home run later in the inning and Alfonso Soriano hit his 21st in the ninth, sending Washington to its fourth straight loss.
Dempster (12-8) has won four of his five starts this month. He wasn't dwelling on Quade's decision to remove him.
"Do I want to come out of the game? No. He knows that -- everybody knows that," Dempster said. "But way too many positive things to come out of today to worry about that, for sure."
Jason Marquis (0-7) remained winless this season despite his best pitching performance of the year. He gave up one run on four hits in 7 1/3 innings, walking three and striking out two.
Marquis has lost all seven of his starts this year. After he walked Colvin, Tyler Clippard relieved with one out and the Cubs scored three times.
"It's frustrating," Clippard said. "I threw a really, really bad changeup up and out over the plate and he hit a double."
Marquis was making his fourth start since returning from a nearly four-month stint on the disabled list and having surgery to remove bone chips from his elbow. It was the first time this season he had pitched more than five innings, and he lowered his ERA from 11.39 to 8.79.
"That was Jason right there. Attacking the zone, aggressive, conviction. It started last game and day by day it's just gotten more comfortable," Marquis said. "It was definitely exciting to get that feeling back, and hopefully I can continue to do it."
The Nationals have lost five of six.
"We're suffering," Riggleman said. "We're getting beat. I don't like getting beat, I'm sick of it and I know our players are."
Nationals RHP Stephen Strasburg, on the DL with a right strained forearm tendon, will have a second MRI Thursday to determine the extent of the injury. His scheduled spot in the rotation Thursday will be filled by RHP Jordan Zimmermann. Zimmermann has not pitched in the majors since undergoing Tommy John surgery last August. After the game, the Nationals optioned RHP Collin Balester to Triple-A Syracuse. ... Nationals OF Josh Willingham had left knee surgery Wednesday and is unlikely to play again this season.