The teams were scheduled to play Monday through Wednesday, but it changed to a Tuesday through Thursday blueprint after rain postponed the opener.
Lopez bested young lefty Ross Detwiler (1-2), who was charged with three runs on seven hits with five strikeouts and two walks over five frames. Jayson Werth hit a solo home run in the loss, snapping Washington's two-game winning streak.
Dempster, who was 5-6 on the season after a 4-3 loss to the New York Yankees on June 18, won his next two decisions, lost a pair and has since won another two straight against St. Louis and Cincinnati on July 31 and Aug. 5.
In those two starts, the 34-year-old Canadian has allowed five runs on 11 hits in 12 innings while walking seven batters and striking out 11.
The Cubs are 8-3 in his last 11 starts and Dempster is 8-1 lifetime against the Nationals with a complete game and two saves in 22 appearances -- 14 of which have been starts.
His earned run average against Washington is 2.97, nearly two runs better than his 2011 mark (4.87) and significantly better than his career numbers (4.41).
For the visitors, right-hander Jordan Zimmermann faces the Cubs for the third time in his career and again aims for his first win.
The 25-year-old Wisconsin native is 0-1 in the first two career meetings with the Cubs, having allowed 14 hits and nine runs -- eight earned -- in 11 1/3 innings.
He last faced them on July 4 in Washington and got a no-decision in the Nationals' 5-4 win after allowing eight hits and four runs in six innings.
Zimmermann is 3-3 in his last 10 starts and prevailed in his most recent outing, defeating Colorado, 5-3, on Aug. 5.
The Nationals have won four of six meetings with Chicago this season after the Cubs took four of six matchups a year ago.