There is simply no valid debate to the fact that Nader cost the Democrats the '00 election. There is no other way to interpret the numbers. It's like trying to claim that 2 plus 2 equals five.
As for Perot, he was far more centrist than Nader is, and he drew support from the center-right and center-left. Much of the weirdo-American community, which is well represented on both sides of the fence, voted for him. As a result, the case that he cost Bush the '92 election is far less clear cut.
Even so, it's possible he did do so, and that possibility is one reason Republicans were such remarkably sore losers for the duration of the Clinton Administration.
No amount of denial will change one basic fact: The United States is a two-party system, and any third party candidate functions only as a spoiler.
You want four more years of reckless lie-based wars (what's next, Syria? Iran? Korea?), unilateral foreign policy, ballooning deficits, insane religious ideologues in critical posts? Go ahead and vote Nader in November.