The endorsement is the first time in more than 30 years that the city's largest police union has supported a Boston mayor for reelection, the union said.

Boston Mayor Michelle Wu will find herself testifying before a Congressional committee Wednesday with a hostile Republican majority intent on grilling her and three other Democratic mayors on their limited cooperation with federal immigration enforcement.

The city’s largest police union, the Boston Police Patrolmen’s Association, plans to formally endorse Mayor Michelle Wu later this week in her re-election bid, according to two sources close to the matter.

Michelle Wu holds a double-digit lead over philanthropist Josh Kraft in the race for Boston mayor, a new poll from Emerson College finds. The preliminary election to narrow the field down to two candidates is in September.

Boston’s largest police officers' union will endorse incumbent Mayor Michelle Wu for reelection on Thursday afternoon, its leadership said. The Boston Police Patrolmen’s Association said its members voted “overwhelmingly” to support Wu at the union’s most recent monthly meeting.