The game was tied, 2-2, when Pothier scored his third goal of the season.
Tampa Bay got one goal and one assist from Vincent Lecavalier. The other goal came from Paul Ranger, while Johan Holmqvist was beaten three times on 32 shots for the Lightning, who have lost four of five.
Each team has won two games in the season series.
Washington went 1 for 6 on the power play while Tampa Bay was held scoreless on its four power man-advantage opportunities.
Notable deaths of 2014 [PHOTOS]