JON SANTIAGO, a state rep trying to build a broader base of support in Boston for his run for mayor, got a lift on Friday when a union local with strong ties to former mayor Marty Walsh endorsed his candidacy.

Laborers Local 223 of Dorchester is part of the Massachusetts and Northern New England Laborers’ District Council and has more than 800 members in Boston. It was former mayor and now US Labor Secretary Marty Walsh’s union, and it is headed by Walsh’s cousin, who is also named Martin Walsh.

“He’s the best guy for the job,” Walsh said of Santiago.

Santiago is an emergency room doctor at Boston Medical Center and a state rep whose district encompasses parts of the South End, Back Bay, and Fenway neighborhoods of Boston. While Santiago’s background is not in construction, Walsh said the lawmaker stands out from the rest of the candidates running for mayor, including City Councilors Annissa Essaibi George, Andrea Campbell, and Michelle Wu, and John Barros, formerly the chief of economic development for Boston.

“He’s got a good sense for promoting development and the jobs that come with that,” Walsh said of Santiago. “I’m not comfortable with a lot of the other candidates when it comes to development.”