Pominville's power-play goal just 51 seconds in set the tone for a Buffalo team that had lost its last three by a combined score of 15-6.
"We got everybody going early, we got the fans into it, and we got the bench going," Pominville said of the early spurt. "You always want to get that first goal, and for us to get it that early was huge for the team. To generate more chances and score more goals after that was big. We limited their chances, and probably could have put a few more away."
Ryan Miller, making his sixth straight start, finished with 20 saves.
"I thought that was a great example of the type of hockey we've been talking about for a while," he said. "It was quick hockey, move the puck, and support each other. It showed in that first period where we got out of our zone clean, we got into their zone clean, we made things happen and got rewarded for it."
By the time the Capitals were able to get their legs under them and get some offensive opportunities against Miller, it was too late.
"After the Christmas break, it's usually decided in the first period," Washington coach Dale Hunter said. "We made bad decisions and gave up bad goals. It cost us the game."
Buffalo's depth at forward was bolstered by the return of Jochen Hecht (foot) and Brad Boyes (ankle). They each contributed an assist.
Capitals forward Mathieu Perreault was taken to a hospital for precautionary reasons and observation after sustaining an upper body injury. Perreault was hurt after his stick got caught in the boards and jammed into his midsection when he went into the corner to hit a Buffalo player in the third period.
He skated to the bench slumped over -- but under his own power -- and was able to play the rest of the game.
Hunter said after the game that Perreault initially had the wind knocked out of him.
Matt Hendricks scored with 17 seconds left in the second, and Alex Ovechkin netted a power-play goal for Washington, which fell to 1-2-1 in its last four.
Michal Neuvirth started for the fifth straight time, but was pulled midway through the first period after allowing three goals on six shots. He was replaced by Tomas Vokoun, who gave up one goal on 17 shots.
It was the second straight game in which Washington fell behind by a large margin. On Friday, the Capitals were down 3-0 to New Jersey after two periods before rallying with three goals in the third. They eventually lost in a shootout.
"We have to put the blame on us," said Hendricks, who scored his first goal of the season. "We need to become a better team from the start of games."
Ovechkin's goal made it 4-2 with 10:09 left.
After falling behind by four in the first period, Washington broke through on Hendricks' goal late in the second. Standing in front of Miller, he was able to deflect Perreault's shot from the high slot to make it 4-1 after two.
The Sabres struck first with a pair of goals on their first four shots. Pominville opened the scoring when he converted Drew Stafford's pass with a one-timer from the inner edge of the left circle.
Ellis made it 2-0 at 3:10 of the first, slipping a backhand past Neuvirth off a faceoff in the left circle.
Ehrhoff then chased Neuvirth with a harmless-looking snap shot from the right point that fluttered in to push Buffalo's lead to 3-0 with 8:45 left in the opening period.
McNabb made it 4-0 with 5:46 remaining with a power-play goal. It was the first shot Vokoun faced.
These teams meet again Friday in Washington. ... Hecht missed six games with his injury, while Boyes missed the past 13. The Sabres are still without six players due to injury, including LW Ville Leino (lower body) and D Tyler Myers (wrist). ... The Capitals fell to 9-4-2 in their last 15 games against the Sabres. ... The Capitals are 5-6-1 under Hunter, who replaced Bruce Boudreau after Washington's 5-1 loss at Buffalo on Nov. 26.
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Arena: First Niagara Center Location: Buffalo, New York