Backstrom cut Florida's lead to 2-1 with a power-play goal at 2:44 of the second period with a shot from a sharp angle.
Marchment scored 27 seconds later to make it 3-1 at 3:11 with a shot that went five-hole.
After the Panthers killed off a double-minor penalty on Marchment, Lundell made it 4-1 at 15:24 before Verhaeghe made it 5-1 at 17:32 with a wrist shot from the high slot.
"Coming out of the way we played the first period, I thought we did just about everything right and we're down 2-0," Capitals coach Peter Laviolette said. "And then even the start of the second period I thought that we were doing the right things, and they get a third and a fourth, and then I thought our game unraveled from there the last probably seven minutes of the second period, and then the third period we didn't do anything.
"It was one of those games where the score goes to three and then four and it wasn't a reflection for me of the game. But after that, I don't like how we played when it was 4-1. That's the problem that I have with the game."
NOTES:Washington forward Tom Wilson missed the game with a lower-body injury he sustained in the first period of Game 1. Brett Leason, recalled from Hershey of the American Hockey League on Thursday, replaced Wilson in the lineup and played 8:17. … The five goals came one short of the Panthers' playoff record. They scored six goals in Game 1 of the 1996 Eastern Conference Quarterfinals, Game 2 of the 1996 Conference Quarterfinals and Game 3 of the 2021 Stanley Cup First Round. … Lundell (20 years, 214 days) became the third-youngest player to score a playoff goal in Panthers history, behind Radek Dvorak (19 years, 76 days; Game 3 of 1996 Conference Finals) and Ed Jovanovski (19 years, 301 days; Game 2 of 1996 Conference Quarterfinals).