Matthew Jones of USC’s Political Science Department says:
It’s really difficult to make the argument that Ralph Nader cost Al Gore the 2000 election, for multiple reasons. There were only 560 votes separating George Bush from Al Gore. That’s essentially within every margin of error, which when it gets within the margin of error, means that there’s too many other factors that could have affected it to say with any confidence what caused Al Gore to lose and George Bush to win.
Every 3rd party candidate got over 600 votes, which means that if any one of those 3rd party candidates had potentially dropped out and those votes had gone to Al Gore, he would have won too. So once you start to make those arguments, you open up a can of worms that you just cannot put back. Almost half of the Democratic Party voters stayed home, so who’s to say that the people who voted for Ralph Nader, if they didn’t have the choice of Ralph Nader, wouldn’t have stayed home or voted for somebody else anyway?