A rather obvious factor really is that the state is small enough that retail politics can be very effective (one of the reasons why Democrats controlled both Senate seats in the Dakotas for a very long time). Other reasons include the influence of organized labor, public lands and farming issues, the well-organized Montana Democratic Party and their successful GOTV operations, Democratic support among college-educated but also WWC and affluent voters in Missoula and Bozeman, the Native American vote, blue state migrants (particularly from the West Coast), etc. I would add that it is nowhere near as “elastic” (incredibly overused term, btw) as many people are making it out to be, though. The state does have a large base of Independent/unaffiliated voters, but it’s still worth noting that both parties have a fairly high floor and the state’s politics is characterized by both a long progressive and a (more recent) conservative tradition. I’d be very surprised if Tester won with more than 52% this year or if the Republican candidate received less than 45% of the vote, honestly.
Do you think, Montana's closeness to Canada could make it a little bit more liberal/libertarian?