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.
Bush and Frist and Nelson's nominee can make it very clear to Nebraska if they want too. This will be a costly vote for Dems in red states if they vote against it because the conservative wrath will be high. If the GOP wins the vote, then Nelson will get a pass for voting with the Dems. This is why I wouldn't be surprised to see Nelson be the 50th vote if it came to it, ie he could lose his seat over this.
Just ask Tom Daschle.