Martin St. Louis added a short-handed goal and an assist. The Lightning remained three points behind Southeast Division-leading Atlanta, which beat Boston 3-2. Tampa Bay also stayed in sixth place in the Eastern Conference, one point ahead of the New York Rangers.
Tampa Bay had other goals from Ryan Craig, Dan Boyle and Jason Ward, who scored into an empty net with 1.7 seconds left.
Matt Pettinger and Matt Bradley scored for the Capitals, who have lost six in a row. Washington has also dropped nine straight and 22 of 24 on the road.