Dorchester Democrat Martin J. Walsh waves to supporters after defeating John R. Connolly ’95 to become Boston’s new mayor.

UPDATED: November 6, 2013, at 1:46 a.m

State Representative Martin J. Walsh, a Dorchester Democrat, won a hotly contested race to replace outgoing Mayor Thomas M. Menino Tuesday night, edging out fellow progressive Democrat and Boston City Councillor at-large John R. Connolly ’95.

Down in the polls by about 3.5 percentage points at the time, Connolly conceded the race to Walsh shortly after 9:20 p.m. on Tuesday. Walsh won with 52 percent of the vote, to Connolly’s 48 percent.

Speaking at a gathering of supporters in a ballroom at the Park Plaza Hotel in Back Bay, Walsh reaffirmed his commitment to a unified city, emphasizing that the city should be, “One Boston, no matter where you live or who you are.” He also emphasized a commitment to public safety, better schools, and closing achievement gaps.

“You made Boston a place where dreams come true,” Walsh told supporters. “Together we are going to make Boston a place where dreams come true for every child, for every person, in every corner of this city.”

In his victory speech, Walsh said that with the election over his attention will now turn to ensuring a smooth transition. “In the coming days, I will [reveal] a transition plan and work with Mayor Menino and staff to ensure the smoothest possible handover from his administration to mine,” he said.

Although Harvard’s main campus is located across the river in Cambridge, the election has implications for the University. During the campaign, Walsh vowed to abolish the Boston Redevelopment Authority, the body that regulates Harvard’s construction projects in Allston, and replace it with an economic development agency less closely tied to the Office of the Mayor. Walsh has also voiced support for extended hours of operation for the MBTA.

Connolly, a former schoolteacher, campaigned on a platform revolving primarily around stronger schools and safer streets. He advanced proposals to close the achievement gap and create a better system for assigning students to specific public schools.

“I know that the city of Boston wants to close the achievement gap, and I know that the city of Boston wants to end violence, and I know that the city of Boston wants economic opportunity to reach every corner of the city,” Connolly said during a speech to supporters on Tuesday night.

Once considered the clear frontrunner in the race, Connolly faced an unexpectedly strong challenge from Walsh, who drew ahead during recent weeks buoyed by manpower and funds from organized labor groups.

The race had been hotly contested since the spring, when Menino announced that he would not seek a sixth term. Twelve candidates eventually obtained the signatures necessary to compete in a preliminary election, held on Sept. 24, in which voters narrowed the field to Walsh and Connolly, with Walsh winning by about 1,500 votes.

In recent weeks, the race grew increasingly personal as Walsh supporters tried to highlight his opponent’s Harvard degree and portray him as a child of privilege.

“Harvard is funny for any candidate running for office,” said Connolly pollster Chris Anderson after his candidate conceded. “It’s a double-edged sword.”

Connolly supporter John Hanna implied that Walsh had used the Harvard connection to unfairly portray Connolly as out-of-touch. “It was taken and manipulated to make it look like it is unacceptable to work hard,” Hanna said. “To utilize someone’s achievement to their detriment, for lack of a better phrase, it’s sour grapes.”

Connolly—who previously had refrained from lashing out at labor unions—tried to respond by hammering Walsh for accepting a massive influx of last minute cash from organized labor. He has argued that the donations would leave Walsh beholden to labor unions while mayor.

Yet overwhelming support from labor unions ultimately pushed Walsh to a solid victory. Early returns showed Walsh with a close lead from the start that only widened throughout the night.

—Staff writer Matthew Q. Clarida can be reached at matthewclarida@thecrimson.com. Follow him on Twitter @MattClarida.

—Staff writer Laura K. Reston can be reached at laura.reston@thecrimson.com. Follow her on Twitter @laurareston.