The first-place Cardinals (83-66) lead the Chicago Cubs (81-68) by two games in the National League Central. The Nationals (82-66) lead the Cubs by 1 1/2 games for the first wild-card slot.
The Cardinals lost two of three games over the weekend to the Milwaukee Brewers, who are one game back of the Cubs in the division and wild-card races.
How will the Cardinals respond after losing four of their last six games to allow their division lead to shrink?
"Just play, that's what we do," Cardinals manager Mike Shildt told reporters after the Cardinals blew a 4-3 ninth-inning lead over the Brewers Sunday afternoon. "Easier said than done ... that's what professional players do and we have a team full of them. You move on.
"You win a big game, you come back and you're hungry for the next one. You lose a tough one, you come back and fight for the next one. We'll show up tomorrow, I can promise you that."
The Nationals hope to have manager Dave Martinez back on the bench Monday. He fell ill during Sunday's 7-0 victory over the Atlanta Braves and was hospitalized.
"He wasn't feeling good," bench coach Chip Hale told reporters. "Just for precautionary reasons, they took him to the hospital just to see what was going on, but we are expecting everything to be good."
The Nationals will start Stephen Strasburg (17-6, 3.49 ERA), who won his previous start against the Cardinals this season 2-1. He allowed the one run on six hits in 6 2/3 innings, striking out nine batters along the way.
Strasburg is 3-2 with a 2.38 ERA in seven career starts against the Cardinals.
He has done some of his best work down the stretch, allowing just seven runs and striking out 44 batters in 33 innings during his last five starts.
Cardinals starting pitcher Dakota Hudson (15-7, 3.38) was on the losing end of that 2-1 game May 2 in Washington D.C., allowing the two runs (one earned) in six innings.
Hudson is 1-1 with a 3.24 ERA in five career outings against the Nationals, including four relief appearances. Like Strasburg, he has been at his best lately -- going 5-1 in his last six starts with just six runs and 16 hits allowed in 38 1/3 innings.
On the injury front, Cardinals closer Carlos Martinez has been sidelined with respiratory problems. He was unavailable Sunday when John Gant walked three batters while blowing a save opportunity against the Brewers.
"I'm more concerned about Carlos and his health," Shildt said. "I just want to make sure that he's OK and we get out of the woods with him from a medical standpoint. We'll take care of the baseball after that."
Cardinals catcher Matt Wieters (calf strain) is also on the comeback trail. He could be available at some point in this series.
Nationals catcher Kurt Suzuki is still working his way back from elbow inflammation. He was out of the lineup for the seventh straight game Sunday.
Nationals first baseman Matt Adams has been sidelined since Thursday with an AC sprain.
--Field Level Media