Brady Tkachuk and Shane Pinto each had a goal and two assists for the Senators (25-28-3), who had won two in a row and three of their past four. Anton Forsberg was pulled after allowing four goals on 10 shots in the first period. Joonas Korpisalo stopped eight of 10 shots in relief.
"We were just a little flat tonight,” Pinto said. “A tough start kind of put us behind the eight ball. There's no excuses. Just got to play better and start better, so I think that was the main thing.”
Protas gave the Capitals a 1-0 lead at 6:30 of the first period. He took a pass from Mantha on a 2-on-1, cut in front of Forsberg and scored with his backhand.
Carlson extended it to 2-0 at 8:30 on the power play, scoring on a wrist shot from just outside the right circle following a face-off. The goal came moments after a video tribute honoring Carlson for playing in his 984th game with Washington, the most by a defenseman in franchise history.
“It was a cool little moment,” Carlson said. “Especially after all that. I think it was just one of those things you can’t make up. Good things happen sometimes.”
Drake Batherson cut it to 2-1 at 10:12. During a scramble behind the Capitals' net, Tkachuk poked the puck out to Batherson, who shot past a sprawling Kuemper.
Pinto tied it 2-2 at 12:41 when he deflected Travis Hamonic’s shot past Kuemper. The goal was initially waved off due to a high stick, but after a video review, the call on the ice was overturned.
Max Pacioretty gave Washington a 3-2 lead at 18:33, scoring on a wrist shot from the left circle on the power play.