The Cardinals dropped the final two games of their weekend series against the Cincinnati Reds, losing consecutive games for the first time since July 22-23.
They arrive in Washington trailing the just departed Milwaukee Brewers by a half-game and the Chicago Cubs by 5 1/2 games in the National League Central. The Cardinals (76-61) had won 10 straight series.
Flaherty went 4-0 in August with a 1.13 ERA and 38 strikeouts in 32 innings. Last time out he allowed one run and four hits in seven innings of a win over the Pittsburgh Pirates. He struck out five without a walk.
"He's a little different. Motivated. Works hard. Takes care of himself. I would really love to see him sustain what he's doing for a long time," Cardinals pitching coach Mike Maddux told the St. Louis Post Dispatch. "He's really learning himself. And the more he learns himself, the more he likes himself, and it shows in his performance,"
Scherzer, a candidate for a third straight NL Cy Young Award, is coming off two starts against Aaron Nola and the Philadelphia Phillies. He lost the first one and got a no-decision as Washington won the second. Scherzer was 2-1 during in August with a 1.89 ERA.
He leads the league in strikeouts and his next one will give him 250 for the fifth straight season.
Scherzer is 2-4 with a 2.70 ERA in eight starts against the Cardinals.
On Sunday, the Cardinals loaded the bases with no outs in the ninth inning but failed to score, and Cincinnati's Eugenio Suarez clubbed a two-run homer in the 10th in the Reds' 6-4 win.
"We weren't able to execute our situational hitting where we wanted tonight," Cardinals manager Mike Shildt said. "It's been a strength of our club, and we expect it to be a strength of our club. We can't overreact to (three) at-bats."
Cardinals right fielder Jose Martinez struck out four times. He has fanned seven times in nine at-bats.
"I think what we're seeing is he's human. Just expanding the zone, maybe trying to do a little too much," Shildt said. "He'll be just fine. We'll take him up there tomorrow in the same situation. I'll tell you that."
Washington (68-69) couldn't build on Saturday's come-from-behind win and lost the rubber game to Milwaukee 9-4. The Nationals walked 11 batters Sunday, matching the team record since the club moved to Washington in 2005.
Nationals starter Jefry Rodriguez (2-2) issued seven walks and allowed seven runs in 4 2/3 innings as the Brewers broke the game open with a seven-run fifth.
"It's the walks," Nationals manager Dave Martinez said. "When you fall behind, it's tough. They got pretty good hitters."
Three games into a 10-game homestand, the Nationals are 8 1/2 games behind the first-place Atlanta Braves with 25 games to play and eight back in the wild-card race.
"You never give in to the game," Nationals second baseman Wilmer Difo told the Washington Post. "Today, it didn't happen for us, but it wasn't because we changed our mindset or approach at all. It's not always going to happen."