If you’ll read my post above, you’ll see exactly why he wants to switch in order to get reelected.
It should be pointed out that he has always been very moderate, and has been able to get as many Democratic votes as Republican. It makes sense for him to switch, especially in an environment where the Republicans in Pennsylvania still skew very strongly to the right. The middle of the state is part of the Bible Belt.
Anyway, that having been said, there’s no certainty that he can get throught the Democratic primary, either. I don’t know who will be up against him, but there are many more progressive Democrats who could run, and it would be nice to have someone who leans towards the left, instead of all these centrists or right-wingers we’ve been saddled with.