Boston Mayor Michelle Wu is kicking off her campaign for re-election with a strong lead over her opponent Josh Kraft, according to an Emerson College poll released Thursday.

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.

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 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 survey offers the first early look at the matchup between the first-term progressive mayor and her most prominent challenger.