Washington, which ended Ottawa's four-game winning streak, has won three in a row at home for the first time since Feb. 24 to March 2, 2003.
After averaging only 1 1/2 goals during a six-game winless drought, the Capitals have averaged 5 1/2 goals in victories over Tampa Bay, Dallas, Buffalo and Ottawa.
The Capitals went ahead 2-0 in the first period on goals 36 seconds apart by Matt Bradley and Clark, who added his second of the game and ninth of the season 13:54 into the second period.
Alexander Semin, Matt Pettinger and Brooks Laich padded Washington's lead in the third.
Jason Spezza and Mike Fisher scored for Ottawa, which played without captain Daniel Alfredsson, injured in Tuesday night's win against the New York Islanders.