I've always looked down on people who say that it's a waste to vote for a third party. I think they're missing something fundamentally important so let me break some news for everyone:
You vote does not matter.
Don't worry, it is counted, but it doesn't matter. I'm not one of those tired and dim non-voters. But If you had died the first time you walked to your local polling place, the outcome of every election thence would still have been exactly the same. Even if you industriously vote at every occasion, the chances of you ever even affecting the outcome of a local election are essentially nothing and the chances of you casting the deciding vote for the President of the United States is practically non-existent. There's a better chance of you being stuck by lightening on your way there and your dead corpse buying a lottery ticket and winning than you deciding the president. It won't happen.
A lot of people say that it's unfair in a democracy that individual people cannot sway elections. They're wrong. If you could actually decide the outcome of an election, it wouldn't be a democracy, it would be a dictatorship by you. You have to share decision making with all other people in society, which ironically enough makes everyone equally powerless.
So Why Not Vote for Someone You Actually Like?
But somehow people convince themselves that if they dedicate their negligible sliver of electoral power to a winning or almost winning candidate suddenly they have somehow made a difference in the world. They haven't. I'm somewhat disposed to say that this is a kind of mental tribalism that psychically rewards every participant in the winning team, but the fact is in modern democracy, it doesn't matter if you're on the winning or losing team, so long as you vote for a candidate whom you think to be genuinely better.
"Don't Blame Me I Voted for Nader"
The "conventional wisdom" would be to vote for Gore so you can push him closer to victory, but as it happens you voting for Gore doesn't change anything at all: Bush would still win by 536. Your vote is still powerless. You may as well just vote for the candidate you believe in most and who brings you the most personal pleasure by casting him your vote.
Nader voters find themselves in a kind of Prisoners' dilemma. They all may feel best if they vote for Nader, but they all know that that won't transfer into a non-Bush victory. A Nader-lover wants to convince other Nader-lovers to vote for Gore to produce a far better social outcome than a Bush victory, but each one of them, knowing that they essentially cannot change the outcome by themselves wants to vote for their one true love, Nader. They want to be Nader-voters in a world where Gore wins, but because of that, Bush becomes president instead and everyone loses.
Dealing with Uncertainty
All this said, anyone could see that people often, even usually do change their vote to more consensus candidates. After all, every voter doesn't just vote for himself even if he shares all his own ideals. What really could change a Nader-voters vote is uncertainty: they may agree with the above analysis but they may be focused on the fact that their voting power really can make the difference in some inconceivably rare circumstances.
I would think that changing a vote to match a mainline party candidate is a kind of insurance policy against the potential disaster of a Nader voter failing to cast the wining vote for his second best, Gore. Many voters who would vastly prefer to vote for third parties are paralyzingly afraid of being personally responsible for allowing for the election of their least-preferred candidate.
Aside from that, the only other reason a Nader supporter would vote for Gore that I could think of would be that the voter naïvely assumed that if he followed the logic of voting for Gore, thousands of other Nader supporters would do it as well because of some kind of spiritual link between them all or something of the sort. But I think anyone who acknowledges that every voter's choice is independent and impotent should just end up voter for whom they genuinely prefer to be president, even if they have no chance of winning.
This applies to every state besides those hypothetical ones made of one or two people. One of the hokey things that people here in my state complain about is that somehow their votes don't matter because this is a stable red state, as if they suddenly moved over to Ohio or Florida their votes would matter. But being generous, the statistical difference between one swing-state vote and one stable-state vote vacillate between insignificant and non-existent.