Anthony Duclair and Brandon Hagel scored for the Lightning (44-28-8), who have lost three of four (1-2-1). Andrei Vasilevskiy made 30 saves.
“I think we just could have been a little more desperate,” Lightning defenseman Darren Raddysh said. “[The Capitals] are trying to get into the playoffs and they played right until the end and we kind of let up a little bit and they took some momentum and they capitalized.”
Tampa Bay will enter the playoffs as the first wild card in the East.
“These last two are just going to be all about details and the little things and doing things the right way because when playoffs come, there’s not much room to make mistakes,” Hagel said.
Carlson gave the Capitals a 3-2 lead with a power-play goal at 8:29 of the third period, scoring on a high wrist shot from the right circle off a pass from Dylan Strome shortly after a 5-on-3 advantage ended.
“To not be deterred just because we didn’t score on the 5-on-3 … to not just throw in the towel, it’s a big, big moment in that game,” Capitals coach Spencer Carbery said. “Even though we don’t score on that 5-on-3, to find that one on the back half was obviously the difference.”
Dowd knocked in a loose puck in front at 16:46 for the 4-2 final.
“There was 20 minutes to play, and the game was tied, so dig in here for a bit,” Lightning coach Jon Cooper said. “And we just made some mental errors. One was on the penalty and then the fourth goal is you just can’t make the plays we made in the D-zone and allow that kind of stuff to happen.”