The polling firm says that Tester added nearly 10 points to his base vote, with 92 percent of Democrats now saying they'll vote for him in November, up from 84 percent last month. Tester is also pulling votes from 20 percent of Republicans, according to the poll.

A majority of voters (54 percent) have an unfavorable opinion of Burns, with more than one third of respondents (37 percent) reporting "very unfavorable" views of the incumbent. Both of those numbers are up 2 points from Rasmussen's last survey. Tester, on the other hand, earns favorable reviews from 55 percent of voters.

This new poll contradicts two other recent polls that found the candidates about neck and neck. One taken last month by Rasmussen found the candidates tied at 47 percent. And in early September, a USA Today/Gallup poll found Tester and Burns closely matched, with 48 percent of likely voters supporting Tester and 45 percent with Burns.

In the fall of 2005, Burns had a double-digit lead, according to Rasmussen. But after the scandal involving disgraced lobbyist Jack Abramoff erupted and entangled Burns, who received contributions from Abramoff and his clients, Burns's popularity plummeted.

–Danielle Knight