Lori Trahan, a onetime aide to former Rep. Martin T. Meehan, is poised to go to Congress from Massachusetts’ heavily Democratic 3rd District after winning the Democratic primary in a recount Monday.

Trahan had a 139-vote advantage when second-place finisher Daniel Koh conceded to her hours before a 5 p.m. deadline.

“There’s no use in getting upset about the close margin — we can’t afford to,” Koh said in an emailed statement. “It’s time for us to unite behind Lori Trahan to be sure this seat stays Democratic.”

Trahan declared victory the night of the Sept. 4 primary in the 10-way race for retiring Rep. Niki Tsongas’ open seat. She will face Republican Rick Green in November.

Koh, a former chief of staff to Boston Mayor Martin J. Walsh, requested the recount after coming in 122 votes behind Trahan in primary.

Correction 5:43 p.m. | An earlier version of this story misstated the first name of the Republican nominee in Massachusetts’ 3rd District.