Nelson might vote with the GOP if his vote is needed to make the rule change. If he doesn't he would have a hard time winning relection in a very red Nebraska. If Frist has the votes, than Nelson will likely vote no and then vote for all of Bush's judges on the floor.
Don't you think that Ben Nelson would gamble on the point that the voters of NE cannot understand this issue -- too complicated for most people. Plus the GOP may be very unpopular in NE in 2006, like in 1974.