I don't think any MT Republican senator has ever won a fourth term, maybe prior to 1930 but doubtful. MT is a very Democrat state except for President since 1952.
Burns will have a better chance to win if the race is nationalized somewhat by it looking like the Senate could tip if they elect the Dem Tester. If the Senate appears safe for the GOP on election morn, then it is in Tester's favor, IMHO.
Burns was the first Republican Senator in MT reelected
period since statehood, including when they were elected by the legislature.
Clinton won MT both times, and JFK won Montana. So there have been Democrat victories prior to that, and Democrat Senators in both of Montana's seats before Burns. Mike Mansfield probably being one of the more prominent ones and has a room named after him in the Capitol. Burns may lose because the GOP deems this race unimportant. The national media sure hasn't been covering it much. At least all that I know of.
There's a lot at stake and, Burns being in the Senate 3 terms is the lease of the GOP's problems. There are bigger problems facing this country, like, Democrats in control of the legislative branch to 2008.