Converted to a starter this season after he was Chicago's closer last year, Dempster improved to 12-2 in 16 home starts. The right-hander, who predicted in spring training that the Cubs would win the World Series for the first time since 1908, is not putting any emphasis on reaching 20 wins.
"I don't know. I'm not worried about that. It's a great personal accomplishment, but my motivation is one reason and one reason only and that's to get to the playoffs and win the World Series," Dempster said. "I will take 15 wins and a World Series and I'd be the happiest player in baseball."
On a muggy day, Dempster retired 13 of 14 batters between the second and sixth in conditions similar to his previous win Sunday at Florida.
"It was as hot today as it was in Florida. At least as humid. It was sticky out there," Dempster said. "But I felt like I got stronger as the game went on. I was able to maintain myself."
The Cubs, who have the best record in the NL at 79-50, bounced back from Friday's 13-5 loss to the struggling Nationals by getting 13 hits and another impressive performance from Dempster, who also won 15 games for the Marlins in 2001.
"Outside of Brandon Webb, there aren't too many people pitching better than he is," Cubs manager Lou Piniella said.
Dempster scattered eight hits, struck out four and walked one, sending the Nationals to their 13th loss in 15 games.
"He's been our MVP," DeRosa said. "I know he set a personal best, but he is not satisfied. He is one of those guys that has busted his tail to put himself in this position. He's been just unbelievable for us."
After stranding eight runners through the first three innings, the Cubs didn't let Washington starter Odalis Perez off the hook in the fourth. With one out, Perez walked Soriano and he advanced to third on Ryan Theriot's bloop single. Derrek Lee followed with an RBI single and Ramirez homered to put Chicago up 4-0.
Ramirez is 7-for-15 against Perez with three home runs and eight RBIs.
Perez said he felt sick before the game.
"I didn't have it today. When I got to the ballpark I started feeling weird in my stomach," he said. "I understand that's not an excuse."
Perez (5-10) lasted 4 1/3 innings. He allowed five runs, four earned, and 10 hits while walking four and striking out four.
"You can only work out of so many jams against such a good ballclub," Nationals manager Manny Acta said.
DeRosa homered off reliever Garrett Mock in the sixth. It was his 17th of the season and the first time he's hit three homers in three games since 2003 with Atlanta. All coming on the first pitch.
Ramirez hit his second three-run shot in the eighth off Marco Estrada. The opposite-field drive to right gave him 22 homers this season and 21 career multihomer games.
The slumping Fukudome went 0-for-3 and struck out with the bases loaded to end the third. He also made two errors on one play in right field. Elected a starter for the NL All-Star team, Fukudome hit .296 through June but is batting .199 since July 1. "I'm going to have a talk with him (Sunday) and see how we can help the young man. He is struggling," Piniella said. ... After striking out in the fifth, Lee was examined by a team trainer. He stayed in the game, then left in the seventh with back spasms. He is day-to-day.