Discussing the politics of this situation is wrong but I am not going to lie and say that I haven't thought about it. In all honesty, though, don't anyone think that a Republican would be appointed anyway. If this situation takes a turn for the worst and there must be an appointment, I bet Rounds will be under a lot of pressure (from South Dakotans and others) to appoint a Dem. Now he could ignore it since he can't run for a third term but I wouldn't be surprised if he went with a Dem.
I'm not sure if the pressure from South Dakotans would be enough for Rounds to appoint a Dem. Probably the majority of South Dakotans would prefer a Republican Senate anyway. On the other hand, a majority of South Dakotans voted for Stephanie Herseth, so I guess if Rounds were to appoint Herseth, a lot of people would be satisfied.
Of course, we should all hope that Sen. Johnson gets better and that the political ramifications would be irrelevant.
