But I don't understand this motivation. I mean, isn't it the case that Bush (should he unfortunately happen to win the election) has to step down in 2008 whatever happens?

The argument is that another four years for the current regime to become entrenched leaves them sufficiently solidly in, and with sufficiently well-established methods of democracy-circumvention, that it will be impossible to vote them out in 2008.

Myself, I wonder whether this point is not already past.