As the primary election for the Middlesex, Suffolk, and Essex State Senate seat approaches, three-month incumbent Sal N. DiDomenico has garnered a slightly weightier list of endorsements from labor unions and local politicos than has challenger Timothy R. Flaherty.

Cambridge officials, however, are split between the two Democrats—Flaherty, a homegrown lawyer, and DiDomenico, an Everett resident with Cambridge roots.

On Friday, Flaherty announced that the Office and Professional Employees International Union, Local 453—which represents over 300 employees of the Massachusetts Bay Transportation Authority—has endorsed his candidacy.

Flaherty has also been endorsed by several law enforcement unions, including the International Brotherhood of Police Officers and the Masschusetts Police Association.

DiDomenico, the incumbent Senator who won the seat in a special election last May, boasts endorsements from organizations including the American Federation of Teachers Massachusetts, Massachusetts branch of the American Federation of Labor and Congress of Industrial Organizations, and the Sierra Club. Two Senators and one Congressman who represent Massachusetts in Washington, D.C., along with State Senate President Therese Murray, have proclaimed support for DiDomenico.

Meanwhile, Flaherty’s spokeswoman Dorie R. Clark mentioned the endorsement of Jarrett T. Barrios ’90, who held the same seat from 2003 to 2007, as “one of the major turning points in the campaign.”

Among Cambridge’s City Councillors, four have endorsed DiDomenico—a former Everett Councillor—and three support the candidacy of Cambridge resident Flaherty.

“I think he’s shown a great deal of energy and focus about the issues here in Cambridge, and that has impressed me,” said Cambridge City Councillor Sam Seidel of Flaherty.

He added that Cambridge may be better served by a representative who is a denizen of the city.

“I think your sense of what happens in the world really starts at home and then it goes out from there,” Seidel said. “It’s important to me that our representative on Beacon Hill really puts Cambridge right up there.”

Chris J. Keohan, DiDomenico’s campaign manager, said that his candidate spent the first 30 years of his life in Cambridge, a fact that he plans to emphasize in mailings to Cambridge residents in the week before the election. Keohan said that while DiDomenico currently resides in Everett, the endorsements he has received from Cambridge political figures demonstrate “the progressive values that he shares with the city that basically shaped who he is.”

Cambridge School Committee member Marc C. McGovern said that he endorsed DiDomenico because of the candidate’s advocacy of early childhood education, job training, and the establishment of a BLGT commission during his brief tenure in office so far.

“When the person is doing a good job, you don’t switch them out,” McGovern opined. “In three months, he has been able to get things done.”

The Senate seat was vacated in January by Anthony D. Galluccio upon his imprisonment for a probation violation. Six Democrats ran for the seat in a special primary held last April; DiDomenico was the winner, closely followed by Flaherty.

The seat is up for grabs again this fall, following the regular election schedule, and only Flaherty has challenged DiDomenico in the Democratic primary. Barbara T. Bush is the only Republican candidate for the seat.

—Staff writer Tyler G. Hale can be reached at thale13@college.harvard.edu

—Staff writer Julie M. Zauzmer can be reached at jzauzmer@college.harvard.edu.

This article has been revised to reflect the following correction:

CORRECTION: September 7, 2010

An earlier version of the Sept. 7 news article "Dems DiDomenico and Flaherty Compete for Endorsements" stated that  there is no Republican candidate for the Middlesex, Suffolk, and Essex State Senate seat. In fact, Barbara T. Bush is the Republican candidate.