He doesn't want to leave Montana. If that isn't a retirement announcement I don't know what is. I guess you can say good bye to the Clinton/Schweitzer ticket now.
I always thought his being a senator made her less likely to pick him. Unless she had no other good options, why would she open up the seat of a Democratic senator in a red state? But even more than that, Schweitzer has an independent streak and doesn't exude the loyal team player vibe Hillary is sure to be interested in. I myself don't think that rules out an Obama 2008 supporter like Tim Kaine. But it'd take a lot for her to pick someone with a risk of going off script like Schweitzer probably comes with.
It doesn't open up a sets. Montana's governor, Steve Bullock, is a relatively liberal Democrat. And a popular one at that. He'd appoint one of the many also popular statewide Democrats to the seat.