The poll, conducted by Princeton Survey Research Associates, shows nearly one in three Brown backers say they could change their mind before Election Day, compared to just 19 percent for Warren. But the poll, which started a week after the Democratic National Convention, finds no evidence of a “bounce” for Warren.

There also is some troubling news for the well-financed Warren campaign. Despite spending millions of dollars to tarnish Brown’s image, the GOP incumbent’s popularity has actually increased in the past nine months.

Brown is now viewed favorably by 57 percent of registered voters, up nine points from a UMass Lowell/Boston Herald poll conducted in December 2011. Brown’s unfavorable rating actually has dropped six points to 29 percent. He is also drawing 22 percent of voters who say they will vote for President Obama.