Tester leads Rehberg by 45 to 43 percent in the poll, the latest to show a tight race in the GOP-leaning state.

Both candidates have been dinged up from the heavy outside group spending on negative ads. Rehberg is viewed negatively by 52 percent of voters, with 42 percent liking the job he's doing, while 48 percent of voters are unhappy with the job Tester has done to 46 percent who approve.

The presidential race is surprisingly close in the GOP-leaning state, according to the poll: President Obama trails Mitt Romney by 50 to 45 percent. That's only slightly worse than how Obama did there in 2008, when he won 47 percent of the vote.

The poll of 656 likely voters was conducted from Sept. 10-11 and has a margin of error of plus or minus 3.8 percentage points.