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.
And if Gore had simply won New Hampshire, Florida wouldn't have mattered. The fact is, Gore ran a shockingly ineffective campaign, unable to even win his home state. Sure, Nader didn't help at all, and if all the Nader voters had indeed voted for Gore (say, instead of staying home), maybe Gore would have won, but he does deserve to share the blame in his loss.