Popular Boston Mayor Tom Menino—an influential Democrat many credited with bringing the party's convention to his hometown in 2004—told WBZ this week that he wasn't taking sides in Massachusetts's high profile Senate race.

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.