The Western New England University Polling Institute survey of 535 likely voters found 50 percent favored Warren, 46 percent backed Brown and 4 percent were undecided.
The poll has a margin of error of plus or minus 4 percentage points and was conducted Oct. 26 through Nov. 1 in partnership with The Republican newspaper of Springfield.
The appearance of Weld and Kerry on the campaign trail brought to mind an earlier contest. The two battled in another high-profile Senate race in 1996 when Weld challenged Kerry for his Senate seat and lost.
Both national parties are keeping a close eye on the Massachusetts election as they battle for control of the Senate.
The race is the costliest in state history. As of mid-October, Brown and Warren had spent a combined $68 million on the race.