Comrie, who missed practice Monday because of an undisclosed injury, returned to contribute a first-period power-play goal and added another in the second before a crowd of 18,810 at Scotiabank Place.
Andrej Meszaros had the other goal for the red-hot Senators (27-19-2), who have now won six of their past seven contests.
Ottawa improved 10-1-1 in its last 12 games and moved past the Montreal
Canadiens, who were beaten by the Vancouver Canucks 4-0 on Tuesday, into second place in the Northeast Division and fourth in the Eastern Conference overall standings with 56 points.
Ray Emery stopped 28 shots for his 20th victory while Tom Preissing added two assists.
With the two points, Alfredsson, who Monday received NHL first star
honours for last week, has 14 during his current five-game point streak.
Steve Eminger, with his first of the season, had a goal and an assist for
Washington (19-20-7), which suffered its third straight loss. Chris Clark also scored shorthanded while Brent Johnson made 29 saves.
The Capitals have just three wins in 11 games and four in the past 14.
The Senators are 10-3-1-1 against Washington since the 2002-03 season.
The Capitals tried to ride Johnson's luck against the Senators and handed
the goaltender his first start in 10 games, since Dec. 26, in place of Olie
Johnson entered the game with a 3-0 record, 0.92 goals-against average and .972 save percentage in his career against the Senators. Johnson had to make a couple of good saves early, including a pad stop on an Alfredsson deflection.
With a little less than four minutes remaining in the period, he couldn't
stop Meszaros' drive from the point.
Comrie made it 2-0 on the power play with 30 seconds to go in the period and the Capitals appeared to be in trouble when defenceman Brian
Pothier, a former Senator, compounded their troubles by taking another minor penalty.
But Clark countered with a short-handed goal with 2.3 seconds left in the period.
Alfredsson jumped on a loose puck at the side of the Capitals' net to score his own short-handed goal midway through the second, Just 67 seconds later, Comrie scored his second of the game from the slot after a pass from Peter Schaefer.
Ovechkin showed some frustration in third. After breaking his stick while on the attack in the Senators' zone, he tossed it to the ice and ran over Ottawa's Dany Heatley from behind on his way to the bench.
No penalty was called and the play gave the Capitals a slight boost. Eminger scored shortly afterward to make it 4-2. Ovechkin drew an assist to extend his point streak to seven games.
Schaefer closed out the scoring with 6½ minutes remaining.
Notes: The Senators were still without centre Jason Spezza and Mike Fisher because of knee injuries. Spezza, who returned to practice Monday, has missed the last 12 games while Fisher has missed the past 10 . . . Centre Kris Beech was a healthy scratch for the Capitals . . . Washington plays at Carolina on Thursday, returns home to face Florida on Saturday, then plays the Hurricanes in back-to-back games Jan. 26 and 27.