A turnover by Campbell led to another goal when he misplayed the puck in front of his crease and Formenton scored five-hole to tie it 2-2 at 13:14.
"I feel bad about those two goals," Campbell said. "I don't want to kill the momentum, and we played so well. I know those types of goals just can't go in."
It was Formenton's second NHL goal, first since Oct. 30, 2018, in his 12 games.
"When I came up here I wanted to be hard on the forecheck, using my speed, and also contributing offensively," said Formenton, who was recalled from Belleville of the American Hockey League on Wednesday. "My main focus right now is my defensive game and making sure pucks are getting out, as a winger."
Forsberg was claimed off waivers by the Senators on March 17. He is the fifth goalie to start for Ottawa this season, the fourth since March 10 (Matt Murray, Joey Daccord, Filip Gustavsson, Marcus Hogberg).
NOTES: Campbell shut out the Calgary Flames 2-0 on March 20 and the Edmonton Oilers 4-0 on Feb. 27 in his prior two starts. ... Spezza scored once at Ottawa in five seasons with the Dallas Stars (March 6, 2016). ... Marner and Matthews have combined to score 22 points (eight goals, 14 assists) in seven games against the Senators this season. ... This was Forsberg's fourth NHL game since March 26, 2018. He signed with the Oilers this offseason, was waived Jan. 11 and claimed by the Carolina Hurricanes the next day, was waived two days later and claimed by the Winnipeg Jets the next day, and was waived March 16 and claimed by Ottawa.