Though Menino, a Democrat, ultimately supported Warren's candidacy, it was initially unclear whether he'd jump in the race to back the member of his own party or then-Sen. Scott Brown (R-Mass.), with whom he has a close relationship. Though Markey has received the backing of a number of prominent Massachusetts Democrats, including Kerry himself, it seems he still has some work to do in the state.

Menino is known for his strong get-out-the-vote machine and relationship with Massachusetts unions, and he put these into action to help boost Warren in November.

And with the Herald reporting that a prominent union leader, Mike Monahan, criticized Markey as a "weak candidate," he might need that extra Menino hand.

Monahan, the business director at the International Brotherhood of Electrical Workers 103, expressed pessimism about Democrats' chances going into the special election.

"The field is weak at best, and in a special election, beating Brown will take a lot of work and a lot of money,” he said, adding that the impending special election "is man-made and it shows the lack of respect that those people have in Washington for the amount of work we did in the Brown-Warren election."