Democrat Jon Tester won election to the Senate in 2006 on a wave of public discontent. Against incumbent Conrad Burns, he won by .9% and with under 50% of the vote. Burns had an approval rating of under 40%.
Now, Tester is up against Congressman and former Lieutenant Governor Denny Rehberg. According to Rasmussen's polling, Rehberg is ahead of the incumbent.
The fact that Tester is significantly under 50% is a real notch against him at this point in the race. The people of Montana have a libertarian streak and strongly oppose ObamaCare. When Tester ran in 2006, he ran as a moderate and then voted for the health care overhaul.