Nader, of course, doesn't have a prayer of actually winning, nor even breaking out of the single digits if he makes it all the way to November.
But nationally, this year is not going to be the "landslide" for Democrats that Nader claims it should be. I think the Democrats have an edge, but it's still going to be close no matter what. And in that environment, if Nader can pull even 2% of the vote away from the Democrats, it may in fact make the difference. Of course, if he gets only the 0.3% that he achieved in 2004, he won't likely be a factor -- depending on which states give him better numbers.
Gotta love Side Show Ralph's tenacity, at least. You'd think after two previous disastrously ineffective attempts, he'd maybe get the message. But he's stuck on this belief that there's essentially no difference between Democrats and Republicans. On some issues he's right about that, but there are plenty of differences on important issues that it really does matter which party is in control (or, ideally, that both parties must share control).
Here's my theory -- assuming that he actually does get on the ballot in a significant number of states, I think he does better than 2004's showing, but not quite as well as he did in 2000. Reason being, he had novelty going for him in 2000, and we had yet to fully understand what direction GWB would take the country. In 2004, there was a more concerted effort to oust Bush (resulting, interestingly, in a greater margin of victory for the President in the popular vote) and Nader had trouble getting on the ballot in all states.
But this time around, after 2 years of a Democrat-controlled Congress, that august body enjoys lower approval ratings than the President himself. A certain segment of the population may have come to believe Nader's assertion that the Democrats and Republicans are essentially interchangeable. Certainly there's significant dissatisfaction with Congressional Democrats after 2 years in the majority, and that may manifest itself in more protest votes from the far left wing for Nader.
If Clinton somehow pulls out the nomination, I guarantee that Nader takes enough votes to hand the election to McCain (though I don't think he needs the help in that case). With Obama, Nader is not likely to be as much of a factor, but still could play a decisive role.
Next up... wait for Ron Paul to decide to run as an independent candidate, and watch 10% of the Republican base swing over to him.