Capuano commended Coakley in a concession speech to his supporters on Tuesday night.
"I want to congratulate the next senator of Massachusetts, Martha Coakley," Capuano said, according to The Globe. "She ran a good campaign, and it was a good clean campaign. I want to be there to make her the next senator."
Coakley will run against state Sen. Scott Brown, R-Mass., for the Senate seat in a general election on Jan. 19. If successful, Coakley will be the first woman elected to the U.S. Senate from Massachusetts, according to the Associated Press.
In his speech on Tuesday, Capuano cited Coakley's "name recognition" and her lack of "slipups" as factors that led to her victory, according to The Globe.
Boston College political science professor Dennis Hale said in an interview with The Dartmouth that while Capuano was a "very credible candidate," he was not well enough known outside of Boston.
"He's basically a Boston guy," Hale said. "He's never campaigned much outside of the city because that's where his district is."
Although Capuano "did the best that he could," Hale said, his campaign was not successful because he tried to convince voters that Coakley is not "really a liberal."
"That just didn't really work, because among all four of these [Democratic] candidates, the difference among them is not very significant," Hale said.
Hale said he said expected Coakley's victory from the start.
"She's the only one of these candidates who's actually been elected to a state-wide office," Hale said. "She's appealing, she talks well, she's smart. She puts people in jail, which is nice if you're worried about crime. And she's got access to an extraordinary women's political network outside the state."
Coakley also defeated Alan Khazei, co-founder of the nonprofit City Year, and Boston Celtics co-owner Stephen Pagliuca, who received 13 and 12 percent of the vote, respectively, The Globe reported.
Despite his defeat, Capuano maintained a sense of humor during his speech in addressing his mother, who just turned 90, The Globe reported.
"Mom, I was looking for a better present," Capuano joked, according to The Globe.
Tuesday's primary saw very low voter turnout, according to The Globe. By 6:00 p.m., just two hours before polls closed, only 15 percent of Boston's registered voters had cast ballots.
Voter turnout is historically low during off-year elections, according to Hale, and the lack of diversity among candidates did little to pique voter interest.
"There's not much excitement if [the candidates] all look alike based on their political positions, and they did," Hale said.
