But... maybe he did, once we consider the notion of a "spoiler" candidate.
You're taught that you must vote for one or the other -- a Democrat or a Republican. Those who charge that Nader was a "spoiler" in 2000 and 2004 are the products of this teaching. The "spoiler effect" and "spiltting the vote" owe their very existence to this notion. The two-party system has been around for so long that people seem hard-pressed to believe that any third-party candidate is "worthy" of running opposite the candidates of either main party -- because there are "only" two parties. These two choices are only considered as the only legitimate choices solely because of tradition. Sure, there are countless "third-parties" in the history of the United States -- but they are almost always overshadowed by the cherished two-party system.
This is what drives the pitiful mentality of "voting for the lesser of the two evils." I suppose this is also the origin of the "spoiler candidate" argument. Voting for a third option "steals" votes from whomever one considers "the lesser of the two evils." Why? Since you think you only have two "legitimate" options anyone who broadens the list of options to anything more than a dichotomy is unwelcome. Uncomfortable, even. But this is only uncomfortable because it directly rejects the notion that there are only two choices. Indeed, a third choice "spoils" this fantasy. Why should it have even come to this? Isn't there room for a candidate who is not an "evil" at all?
I'm really sick of people complaining about Nader. People are repeating this notion again, now that Nader is running. But the more they repeat this, the more they reinforce the two-party system's alreadly untouchable nature.
I also do not think it is a coincidence that third-parties have often advocated for some of the most radical ideas on either side of the spectrum (there goes that human propensity to classify every and anything), and the two main parties have been "centrist" (as of late)... But, I don't know if the latter half of this opinion will stand up to actual American history. I think they are mostly centrist, with only a few issues here and there that end up being the only distinguishable issues between the two. I've done some research on Duverger's Law, and the effects of having a single-member district plurality voting system. Legislative seats are awarded in a particular constituency to the candidate that has received the plurality of the votes within that same constituency, rather than apportioning the legislative seats according to the total votes gained across the entire set of constituencies. This system tends to reward the two major parties and discourage the development of thrid-parties. (Thanks Wikipedia!)
The "spoiler effect" and "splitting the vote" are only legitimate because we all agree to agree that they are. It will only stop when we realize we have other choices.