State Rep. Nick Collins, a South Boston Democrat, is backing City Councillor At-Large John Connolly in the race to replace Mayor Thomas Menino, several sources told the Reporter on Wednesday.

The son of a former state representative, Collins was elected to the Fourth House District in 2010, winning a three-way primary and easily dispatching a Republican candidate after longtime lawmaker Brian Wallace decided against running for reelection. Collins’s district includes South Boston and Columbia Point, which is part of Dorchester.

The Connolly endorsement puts Collins at odds with his uncle’s union. The city’s firefighters union, Local 718, is run by Rich Paris and has endorsed state Rep. Marty Walsh, a longtime Dorchester Democrat and labor leader. Connolly, an attorney and former teacher, lives in West Roxbury.

Earlier this year, Collins unsuccessfully ran for the First Suffolk Senate seat, which opened up after Jack Hart (D-South Boston) took a job with a law firm. He faced off against fellow South Boston Democrat Maureen Dahill, and Linda Dorcena Forry, a Dorchester state representative who won the primary and the general election.

Rep. Walsh, not yet a mayoral candidate, stayed neutral in that race, declining to endorse either House colleague, and Dorcena Forry eked out a win with 10,220 votes to Collins’s 9,841 votes and Dahill’s 1,594 votes. Dorcena Forry is married to Reporter editor Bill Forry.

Separately, Collins last year joined several other lawmakers, including Dorcena Forry and City Councillor Matt O'Malley, in backing a Connolly plan to revamp the city's school assignment system as a task force weighed changes to the system.

Collins could not immediately be reached for comment.

UPDATE: This story was updated at 3:26 p.m. with background information on Collins.