He's had two out of three competitive contests, and politicians who weather these kinds of races regularly often keep winning. Tom Harkin and Jesse Helms would be such examples. Granted, it is not a guarantee, so I agree with those who say we don't know this early out.
Good examples with Helms and Harkin, still, Al D'Amato was nailed on the fourth run. Though one can easily conclude that Montana is slightly friendlier turf for a long-term Republican incumbent.
As of right now I am calling it a toss up.