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.
They'd be angry and would probably want Herseth appointed. Again, I wouldn't be the least bit surprised if Rounds went with a Dem for the seat. I don't think it matters though. Johnson will be fine.