The Nationals got off to a great start against right-hander Randy Wells. Nyjer Morgan led off by drawing a walk, then stole second and third base before scoring on a double by Cristian Guzman.
Ryan Zimmerman followed and lined a two-run homer over the left-center-field wall to give Washington a 3-0 lead.
Morgan ended up leaving the game after that half-inning because of a broken left hand and was replaced by Willie Harris, who will now be the everyday center fielder.
Morgan hurt the hand going headfirst into third base in the first inning. Asked if Morgan should have slid feetfirst into the bag, general manager Mike Rizzo said, "Those are instinctual things. It's very difficult to change when you get to this level of baseball."
Morgan, who was acquired from the Pirates on June 30, has been a key contributor in the Nats' second-half turnaround. He was a force from the leadoff spot, hitting .351 with one home run, 12 RBIs and a team-leading 22 stolen bases.
"It's a stumble for us, but Willie has been out there before Nyjer got here," Rizzo said. "Willie is going to be out there again. It gives him a chance to step up and do what we have him on the team for. The other guys have to step up like we did today."
In the second inning, Nats starter J.D. Martin scored on a groundout by Guzman. An inning later, Adam Dunn hit a solo shot over the right-center-field wall to give Washington a 5-0 lead.
"We had a real good day by a lot of guys," interim manager Jim Riggleman said. "You can't get five runs early and sit on it."
Washington didn't score another run as Wells blanked the Nationals for the next 3 1/3 innings. And the Cubs would make it a close game by the seventh inning.
With Martin on the mound in the third inning, Aramis Ramirez hit a two-run homer, while Derrek Lee hit a solo homer two innings later.
Right-hander Tyler Clippard gave up the Cubs' fourth and final run in the seventh inning when Ramirez singled to center to drive in Jeff Baker.
In the same inning, Lee tried to tie the score on a wild pitch, but catcher Josh Bard was able to throw him out with left-hander Sean Burnett making the tag.
Both Bard and Burnett felt they got lucky getting the out. Bard wasn't sure if he was going to make a good throw because the hitter, Kosuke Fukudome, was blocking their view.
"I thought the throw was going to hit Fukudome on the back. I couldn't even see Burnett," Bard said.
The ball went over Fukudome's head and that's when Burnett saw the ball coming his way. Burnett fumbled the ball before tagging Lee out.
"I had trouble seeing the ball and I tried to make the tag as quick as I good," Burnett said.
Mike MacDogual ended up with a five-out save, his 13th of the season.
"The Cubs are a pretty good baseball club," Burnett said. "They are in a pennant race. It's an intense game, especially playing at Wrigley. It's a little confidence boost. We had some good pitching. It gives us confidence to play a good team like that."
Bill Ladson is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.