The volatility of voters in Massachusetts and the momentum that has been building behind Republican Scott Brown leads CQ-Roll Call to change the rating of the race to fill the Senate seat of the late Edward M. Kennedy to Tossup.

Massachusetts is a predominately Democratic state, and Democratic nominee Martha Coakley, the sitting state attorney general, started the race with built-in advantages that haven’t produced the walkaway victory her party expected.

Political insiders have become increasingly convinced that the race is too close to call, and polling in the state has produced wildly disparate conclusions. On Thursday, one survey found Brown, a state senator, trailing by 8 points and another found him leading by 4 points.

What’s clear from the analyzing the polling data as a whole is that the race has tightened to the point where it is not possible to say one candidate has a clear edge over the other.