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.
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.
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