Senator John Kerry today begins the delicate task of campaigning for Senator Scott Brown’s political opponent while not alienating himself from his junior colleague.

The Massachusetts Democrat will attend a morning campaign event in Somerville and an evening fund-raiser in Boston for Elizabeth Warren, the Harvard Law School professor challenging Brown for reelection.

In between, Kerry and Warren will speak at a labor rally alongside AFL-CIO President Richard Trumka.

Kerry previewed this political challenge earlier this month when he hinted he would be endorsing Warren soon. He avoided a question about whether Brown’s political record — which has led the Republican to be ranked by one publication as the second-most bipartisan member of their chamber — offered just the kind of political compromise the senator says is needed in American politics.

“What I want to do is first explain why I’m supporting Elizabeth, and I will do that somewhere in the next few days, and then we will go into the differentials and other pieces of it,” Kerry told reporters after he addressed the Massachusetts delegation to the Democratic National Convention.

Sen. John Kerry, speaking in Somerville Monday morning, said control of the Senate, and as a result, control of the national agenda, is at stake in the race between Sen. Scott Brown, a Republican, and his Democratic challenger, Elizabeth Warren.

Kerry, speaking at Davis Square's NGP VAN, a tech firm that works on Democratic campaigns, went out of his way to say nothing bad about Brown, his senatorial colleague.

He did say that Brown, if reelected, could be the deciding vote to hand control of the Senate to Republicans, which would have consequences for the country.

Kerry referred to environmental policy, foreign policy and Supreme Court nominations as areas where the Republican party differs from the will of Massachusetts voters.

A Senate controlled by Republican Mitch McConnell as opposed to Democrat Harry Reid would be bad for Massachusetts voters, Kerry said.

Somerville Patch will provide video of Kerry's talk in Somerville shortly.