The NL Central-leading Cardinals have won nine of 10 and lead the division by six games. They've won nine in a row over Washington, coming from behind the last two nights.
Moss, who struck out with the bases loaded to end the third, homered off Casey Janssen (1-4) to straightaway center on a 1-0 count. Janssen took the loss on Monday, allowing four runs in two-thirds of an inning.
Mitch Harris (2-1) worked the last two innings, allowing two hits.
Ryan Zimmerman homered and Anthony Rendon had a two-run double in a four-run third for Washington.
