So he has no chance in the general?
Despite no Democratic candidate filing in the primaries, there are a decent bunt of Democrats in this district, so I imagine plenty of them will vote for Halvorson without knowing who he is. Add in those who voted for him in the primary, plus possibly some independent voters, and he has a chance. Of course, he said he'd switch right back to the GOP if elected.
Is this why Cook, Sabato, et al haven't put this race on their radar?
Probably. He really did say that he would do that, which makes sense, he would not fit in at all with the Democrats, and he is to the right of many Republicans. Whether the voters know it or not, the race is really between two (different types of) Republicans. Ironically, the one on the Democratic line is running to the
right of the one on the Republican line.