I doubt it. He would probably run for President. I can't imagine Montanans voting for a 2nd Democratic Senator and I doubt Tester will lose in 2018.
They had two Democratic senators up until 2015, and Baucus consistently got reelected since the late 70s. What's stopping them from doing it again? And don't say politics have changed, because if they have, then why did Bullock get reelected?
Sorry but politics have changed. Montana isn't a swing state anymore (remember that Clinton had won Montana). Bullock won for the same reason Baker and Hogan won. But hey I hope I'm wrong.
It's not a swing state, but Democrats can still win state and local races every now and then (Trump won Montana, but that did not bring down Bullock; I can see there being a large group of working class white Trump-Bullock voters; in WV, Jim Justice won an open-seat gubernatorial race; Irvine, CA voted for Hillary Clinton, but also voted for incumbent GOP Congresswoman Mimi Walters and elected Republican Steven Choi to an open Assembly seat). Did you expect Bullock to lose when he ran for a second gubernatorial term?