After forearm tightness twice kept Stephen Strasburg from making a career-high 29th start, heavy rain forced him to wait another day.
While Washington and Miami waited nearly four hours until Saturday's postponement, Cincinnati lost at Pittsburgh, trimming the Nationals' deficit for the NL's second wild-card spot to 4 1/2 games.
"We're not mathematically out of it yet," pitcher Jordan Zimmermann said. "So we've got to keep fighting until the end, and hopefully one of these other two teams tank."
Strasburg (7-9, 2.96 ERA) was shut down after his 28th start last season despite Washington (83-71) being in the middle of a pennant race en route to an NL East title. The right-hander looked like he would easily pass that mark until suffering his ailment Sept. 12 before a scheduled start against Philadelphia the next day.
Strasburg was slated to pitch Thursday's series opener against Miami but felt tightness in his forearm the day before while playing catch.
"He wanted to pitch (Thursday night)," manager Davey Johnson told the team's website. "It was my judgment. I said there's no need. I don't want a guy that has a little discomfort in his arm the day before he pitches. All these games are important, but his well-being is more important than him starting today."
Strasburg will make his first start since a 6-4 win in Miami on Sept. 8. He gave up four runs and four hits over six innings while striking out seven.
Strasburg is 2-2 with a 7.56 ERA in his last six starts against the Marlins after going 4-1 with a 1.56 ERA in his first seven.
The Nationals are 8-0 at home against the Marlins this year after outhitting them 11-2 in Friday's 8-0 victory. Denard Span went 2 for 5 with two RBIs the day after his 29-game hit streak ended.
The Marlins (56-98), losers in 12 of 15, will start rookie left-hander Brian Flynn (0-2, 9.64) opposite Strasburg. The left-hander walked just one Tuesday after issuing nine over eight innings in his first two big league starts, but gave up 11 hits and six runs over six innings of a 6-4 loss at Philadelphia.
Bryce Harper is 12 for 27 (.444) over his last seven games with seven RBIs, but is hitting .218 off lefties this year compared to .315 against right-handers.
Before Strasburg, Dan Haren (9-13, 4.88) will get the ball for Washington, opening a doubleheader for the second straight start. He retired 14 of the final 15 batters he faced while throwing six innings of a 6-5 victory over Atlanta on Tuesday in the first game following Monday's Navy Yard shootings.
After posting an 11.37 ERA over three outings, he's since allowed one run over 12 innings in two starts.
Haren is 0-2 with a 5.40 ERA in three outings versus Miami this year, all on the road, receiving one run of support over 13 1-3 innings.
Starting for the Marlins in Sunday's first game will be Tom Koehler (3-10, 4.51), who is 0-4 with a 4.84 ERA in his last eight starts. The right-hander, however, pitched three-hit ball over eight scoreless innings in a 1-0 loss at the New York Mets on Sunday.
"As far as consistent quality, today was definitely up there," he told the team's website. "There have been some times where I felt like I had thrown the ball maybe like that for a couple of innings. Today was the first time I was able to push there and keep on going."
Koehler didn't fare nearly as well in his only start against Washington. He was tagged for six runs in five-plus innings in a 9-0 loss Aug. 29.
Jayson Werth went 2 for 3 against him with a three-run homer and Harper had a two-run shot.
Koehler enters this matchup after cutting a finger on his left hand while lifting weights.
"He should be fine," manager Mike Redmond said. "It's his glove hand."