"It was probably our most complete game of the season, 60 minutes, all facets," Backes said. "We loved our effort tonight."
The Blues, who entered the game leading the NHL in goals-against average (1.9 goals per game), have now won three straight games and 10 of their last 11 games to improve to 12-4-1 with a Central Division-best 25 points.
"It's just a big team win tonight and a nice way to finish out a home stand," Schwartz said.
The Capitals (7-7-3, 17 points), who were playing the first game of a three-game road trip, lost for the second straight night. They lost to the New Jersey Devils 1-0 on Friday in Washington.
"That's probably the hottest team going right now," Washington defenseman Brooks Orpik said. "The top three lines, they could all be their first line. It's tough to tell."
The `STL Line' of Schwartz, Tarasenko and Lehtera kept rolling with two more goals, giving the Blues a 2-1 lead going into the third period.
Lehtera opened the scoring for the Blues with his seventh goal of the year on a pass from Schwartz with 10:49 left in the first.
The goal was Lehtera's fifth goal in four games and extended his point streak to seven games, giving him 11 points (five goals, six assists). The helper gave Schwartz a six-game points' streak (two goals, eight assists).
Washington's Joel Ward tied the score with 13:22 left in the second period. His wrist shot from the left circle appeared to be stopped by Elliott but it trickled past the Blues' goalie for Ward's eighth goal of the season.
Schwartz put the Blues back in front 2-1 with his seventh goal of the year with a rebound of a Shattenkirk shot with 7:24 remaining in the second period. Shattenkirk and Gunnarsson got the assists.
Shattenkirk's two assists gave him 15 assists and 16 points in 17 games this season.
St. Louis added two goals in the third.
Backes notched his fifth of the season just 12 seconds into the third period. Capitals goalie Justin Peters misplayed the puck behind the net and Berglund slipped a pass to Backes, who buried a quick shot into the wide-open goal.
Then Berglund made it 4-1 on a wrist shot assisted by T.J. Oshie and Backes with 3:19 remaining.
"You have to consider them a contender in the West and you get measured up by them and you see the level of play, the level of physicality and the level of commitment that's needed to be successful," Washington coach Barry Trotz said. "There's a reason they've won 10 of 11 games. They play a pretty solid game."
NOTES: The Blues traded veteran defenseman Jordan Leopold to Columbus on Saturday for a fifth-round draft pick in 2016. St. Louis recalled Chris Butler from its AHL affiliate, the Chicago Wolves, to serve as their seventh defenseman. . The attendance of 19,339 was the Blues' third sellout of the season. . Since 2010-11, the Blues are an NHL-best 42-10-4 in interconference play. St. Louis has won five straight games against the Eastern Conference since losing to the New York Rangers on Oct. 9. . Blues RW Vladimir Tarasenko had a seven-game point streak snapped. . The Capitals did not go on the power play on Saturday, which was the first time this season. They only had one game last season without a power play.