Democratic mayor of Boston refuses chance to endorse Warren

But Menino's reluctance to put the weight of his campaign operation behind Harvard professor and consumer rights activist Elizabeth Warren is a tough blow for the Democrat, who is hoping to upset incumbent Republican Sen. Scott Brown.

"When you vote, it's a secret ballot. I think, I've watched both these candidates, I know Scott Brown real well, I've interacted with him, I've watched Elizabeth Warren also. It's going to be a tough race. It's going to be a very close race," Menino said.

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Menino went on to say that the campaign would hinge on mobilizing voters, noting that Scott Brown needed to turn out Republican and Tea Party voters, while Warren had "the Democratic establishment—for the most part."

The interviewer then asked Menino if he was "with" Warren, noting that the conventional wisdom was that a statewide Democratic campaign couldn't succeed without his support.

"I'm not with anybody at this time. At this time, I'm still not involved in the campaign," Menino responded.

But Menino hinted that he would get involved with the election somehow—either directly or through surrogates—and acknowledged his friendship with Scott Brown.

While Menino is unlikely to endorse Brown explicitly, withholding his support from Warren could have serious impact on the Senate hopeful's campaign. His comments also underscore the difficulty of running for office without prior experience—or alliance-building—in the rough-and-tumble trenches of Massachusetts politics.

Still, Warren held a slight lead over Brown in a Public Policy Polling survey released last week. The poll showed Warren pulling 46 percent in the state, versus 41 percent for Brown.