One may say that "Nader cost us Gore as a president." This assumes that Nader voters would have voted for Gore if Nader weren't on the ballot -- quite an assumption. It has been pointed out that that slim margin in Florida in 2000 would have tipped the other way if the voters for one of the few socialist parties had all voted Democrat in Florida instead.
I read something in a book written by an economist. It is so obvious, I felt foolish not having thought of it myself. Your chance of deciding an election is comparable to that of winning a lottery. You've decided an election if your candidate wins or loses by one vote. Otherwise your vote is futile and meaningless. Before you send me an angry reply, dear climber/voter, I'll state that I'll foolishly continue to vote anyway.