The Astros have a 12-game lead in the American League West, with the second-place Angels still in action on Wednesday night. The Nationals are 13 1/3 games ahead of the second-place Marlins in the National League East.
Nationals starter Edwin Jackson (4-3) allowed two runs, three walks and six hits in six innings. Marisnick broke a 1-1 tie in the fifth when he took Jackson deep to left field.
It was still 2-1 in the eighth inning, until the Astros posted a four-spot. Stassi blasted a 442-foot homer off Matt Grace, and Bregman cranked a three-run shot off Shawn Kelley.