To those who are still whining about Ralph Nader's candidacy I say, "You've all been had...not by the Republicans, but by the Dem Party slicksters who managed to demonize Nader and convince you that Kerry (like Gore in 2000) was 'the only candidate who could beat Bush' in 2004."
Votes are precious things, but all of you smartass 'strategic' voters threw your votes away. If all the self-professed progressives who actually preferred Nader to Gore in 2000 had voted their preference, the Democrats might have fielded a decent candidate in 2004. Instead you proved you'd vote for ANYBODY they nominated, and would be willing to villify a candidate who actually represented your own values and interests. Predictably, the DNC (already assured YOUR votes were in the bag) responded by running the hemming-hawing obfuscator (and fellow Bush 'Bonesman'!) John Kerry.
If progressives had ignored the Democratic Party's scare tactics, if they hadn't allowed themselves to be threatened and cowed into voting for Gore in 2000, if they hadn't disempowered themselves, the resulting Nader vote might have convinced the Dems that their Republican-Lite strategy was a loser....as, indeed, it proved to be (again!) in 2004.
Now do you see how it works? It ain't rocket science....and it certainly ain't beanbag....but if politics is done right we wouldn't have 50% of the electorate staying home out of disgust or despair.