Jeff Carter had a goal and an assist, Trevor Lewis and Kyle Clifford scored, and Jonathan Quick made 25 saves for Los Angeles, which has won four of six games.
The Kings (37-25-5) lead the Colorado Avalanche by one point for the second wild card and trail the Anaheim Ducks by one for third in the Pacific Division.
WATCH: [All Capitals vs. Kings highlights]
"They're all playoff games, for us especially," Lewis said. "Coming down the stretch here, we've got some must-win games just to be able to get in (to the playoffs), so every game is big right now."
Jakub Vrana scored, and Philipp Grubauer made 26 saves for Washington, which has lost the first two of its three-game California trip (4-0 to the Ducks on Tuesday) and is 3-3-0 in its past six.
The Capitals (37-23-7) trail the Pittsburgh Penguins by one point for first place in the Metropolitan Division and lead the Philadelphia Flyers by two for second.
Washington forward Alex Ovechkin remains two goals shy of 600 in his NHL career.
Lewis returned after missing the previous 12 games with an upper-body injury and gave the Kings a 2-1 lead at 9:32 of the second period when he scored on his own rebound.
Lewis came off the bench and took a pass from Carter with room down the left side. Grubauer made the save on his first two shots, but the second rebound hit the shaft of Lewis' stick and went over the goalie's right shoulder.
"I don't know where that came from," Lewis said. "I got it out of the air, that was nice. I saw the first (rebound) and the second one just kind of bounced out and I whacked at it and, luckily, I got a stick on it."