Which GOP Senators are likely to become Blue Dog Democrats?

[Ye We Can]

I don't really understand why any of the current Republican Senators would switch. The majority of switches in the past generation seem to born more from political expediency and less from some untenable divergence of ideology/policy positions. Considering Democrats are probably not going to have any kind of Senate majority until at least 2020 or 2022, it just doesn't make sense to me.

If a few anti-Trump Republicans switch, that would give Democrats a majority in 2017.

I think you are missing my point though. With the GOP at the peak of national+state power since the 1920s, there is no reason for anyone to switch. None of those states are dramatically trending away from Republicans. Collins has no good reason to leave, at least right now. Heller is the only remotely possible candidate for this imo, and just because they are vulnerable in a midterm that probably won't favor them doesn't mean they'll just go switching parties. They could easily get tossed in the primary in that case. It's a silly idea right now.

McCain, Portman, Collins, and Heller don't like Trumpism. What if they feel Trumpism is taking over the party? Why wouldn't they switch to distance themselves from Trump? If they switch, they would become DINOs. They would still effectively be Republicans in practice.

Lol dude, you're missing the point. All of them are popular with the primary electorate of their states (except for McCain, but he won still, and this is probably his last term anyway) They'd all be DOA if they switched in our current political climate. Its not happening barring a worst case Trump presidency.