Mayor Menino to endorse Elizabeth Warren in Senate race at Roslindale Square rally on Friday

Mayor Thomas M. Menino is expected to endorse Democrat Elizabeth Warren on Friday in her bid to unseat Republican US Senator Scott Brown, according to two people with knowledge of the plan.

Menino will appear with Warren Friday at a 4 p.m. rally in Adams Park in Roslindale Square, according to the two people, who spoke on the condition of anonymity because they had not been authorized to discuss the plan.

The news was first reported Thursday morning on the website of the Dorchester Reporter. The endorsement will end months of speculation about Menino’s role in a race that could tip the balance of power in the US Senate.

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Menino may be a Democrat, but he has a history of playing hard to get with other Democrats running for statewide office. Until word leaked of Friday’s endorsement, some political pundits had interpreted Menino’s silence in the race as tacit support for Brown.

Menino and Brown have developed a close relationship, often appearing together at events in Boston. Menino has described Brown as a gifted retail politician who excels at relating to everyday people on the campaign trail. In 2011 Menino went so far as declaring Brown unbeatable. But that was before Warren, a favorite of the national Democratic Party, entered the race.

Warren, Menino said last spring, has “great credentials and understands the issues.” Menino had stated outright that he would not break from his party and back Brown, but he had been coy about lining up behind Warren.

Menino’s personality may mesh with Brown, but his politics align with Warren. Some of Brown’s votes in the Senate have been at odds with issues dear to the mayor, such as supporting budgets that slashed money for public housing, community health centers, and grants for urban social services.

Now the real issue is whether Menino pushes his get-out-the-vote machine for Warren in November. In the past, he has publicly endorsed a fellow Democrat but not followed through with his political muscle.

In October 1998, Menino grudgingly endorsed Democrat Scott Harshbarger’s bid for governor. Menino and Harshbarger clashed before the campaign, and the mayor had a close relationship with the acting governor at the time, Paul Cellucci, a Republican.

On Election Day 1998, Menino’s endorsement of Harshbarger proved largely hollow. His machine remained mostly idle, and Cellucci won.

The other question is how much Menino’s political machine matters in a presidential election year when a surge of voters can double the ballots cast in Boston.