"Asked if he would withdraw if he concluded his candidacy would merely ensure President Bush's re-election, Nader told interviewer Tim Russert, 'When and if that eventuality occurs, you can invite me back on the program and I'll give you the answer.'"

I'd read that as a coded way of saying "Yes" to that question. In which case the idea would be to use a candidacy as a way of injecting some issues into the campaign, and of motivating some people, who might otherwise be inclined to sit on their hands, to get involved, and ultimnately vote to get rid of Bush.