The decision was announced three hours after the scheduled 1:35 p.m. start time. Thursday is the teams' only remaining mutual day off.
Rain fell at Nationals Park throughout the weekend.
The start of Friday's game was delayed 1 hour, 21 minutes. The rain returned with two outs in the top of the second inning and play was stopped. The game was postponed after a wait of 2 hours, 55 minutes when the rain had stopped.
Saturday became a doubleheader that was supposed to start at 3:05 p.m. but didn't get underway until 5 p.m. A between-games ceremony honoring former National Jason Werth and another rain delay in the nightcap led to 1:44 a.m. conclusion as Washington swept the Cubs.
Chicago began Sunday with a 2 1/2-game lead in the NL Central while Washington was 7 1/2 games behind Atlanta in the NL East.
The Cubs, in the midst of 23 games in 23 days, finished a four-city road trip 5-5.
"So many (games) in a row, the four cities, the weather, this is the kind of stuff that can actually make you better as you move forward," manager Joe Maddon said. "As long as nobody's crying and complaining, these are those things that can make you stronger."
Washington snapped a three-game skid with Saturday's sweep, which was led by their ace hurler and all-star slugger. After Max Scherzer tossed a complete game in the opener, Bryce Harper's two-run homer capped a three-run rally in the seventh inning of the nightcap.
"The best way I could describe it is perseverance," manager Dave Martinez said. "For those guys to sit there all day ... and then go out there and do what they do, it's a testament to what kind of team we have."
THE WAIT IS OVER
Cubs OF Terrance Gore singled off Scherzer in the opener Saturday for his first career hit in his 55th game. Gore was used primarily as a pinch runner during parts of four seasons in Kansas City (2014-17). It was only his 16th career plate appearance.
Gore has 25 stolen bases in 29 career regular-season attempts, including four with the Cubs this month.
"He's definitely an interesting cat to have on the team," Maddon said. "If you ever went to a 26-man roster, he might be the perfect fit during the regular season."
Cubs: RHP Brandon Morrow (right biceps inflammation) threw a bullpen session on Sunday.
Nationals: RHP Jeremy Hellickson (right wrist sprain) will throw an 80-pitch simulated game this week while the Nationals are in Philadelphia.
Cubs: LHP Jon Lester (15-5, 3.53 ERA), whose Friday night start was washed out after one inning, opens a three-game home series against the Brewers, their closest pursuers in the NL Central. Lester is 4-2 with a 2.40 ERA in seven career starts against Milwaukee.
Nationals: RHP Erick Fedde (1-3, 6.00), Sunday's scheduled starter, will be pushed back to Monday against the Phillies as Washington opens a three-city trip that will be broken up by a visit home for the makeup game against the Cubs.
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