How could a vote for Nader help Bush, if it wouldn't have made any difference to the result if it had been switched to Kerry anyway , because the margin in a particular state between Bush and Kerry was always going to be much bigger than any conceivable Nader vote.

I suppose it's possible to argue that anything can happen in an election, and it might turn out that it was Florida-close in what had been assumed to be a safe state. But Florida was always expected to be a close race.