Glendening gave the Lightning a 1-0 lead at 7:29 of the first period, deflecting Hedman’s shot from the left point.
The Capitals tied it 1-1 at 2:01 of the second period when Nick Jensen’s slap shot from above the right face-off circle deflected in off Mantha at the right post.
It was Mantha’s fifth goal in his past eight games and 10th in 27 games this season. He scored 11 in 67 games last season.
“I came into this year with a mentality to jump back on the train and it was a hard year last year,” Mantha said. “I was focused, determined to step up this year and that’s what’s going on right now.”
Jensen had a goal overturned at 8:39 after Tampa Bay challenged for offside.
“They’re hard to get through,” Cooper said of the Capitals. “So it was good to get that one early and, obviously, helped by the offside challenge, but two teams that play pretty tight and, hence, a 1-1 game and it goes to a shootout. But it was one that we’ve been on the wrong side of this year and it’s good to be on the right side of it.”