A long-standing principle of online debate holds that the first person to invoke Hitler or the Nazis loses by default. 
The reasoning here is pretty obvious: such comparisons are almost always ludicrously overblown. The fact that a person is wrong, or mean-spirited, or even sociopathic, does not mean he or she is on a par with the historic figure now widely regarded as the living embodiment of pure evil. Such constructions as "feminazi" and "Hitlary" are not merely foolish; they grossly trivialize the actual horrors of the Third Reich.
I propose, by analogy, that we adopt a clear and straightforward standard by which to reject, out of hand, a certain type of comment that's becoming more and more frequent on political blogs. Specifically, I suggest we automatically dismiss any statement that takes the general form:
"If Candidate X is nominated, I'm going to vote for [McCain, Nader, Putin, Pedro, etc.]."
Such comments should be ignored -- and the person who posts them should be considered to have lost several points of credibility -- because:
1. If the commenter is serious -- if he or she truly would cast an anti-Democratic vote, knowing what's at stake in this election -- then the poster has no credibility to start with. Whether it's a case of trolling, or irrational thinking, or political immaturity, or sheer vindictiveness doesn't really matter.
2. If the commenter is not serious -- if this is just an emotional outburst, or melodramatic posturing, or overstatement for effect, or an outright lie -- then why bother to argue? The poster will either act responsibly in November, or not. 
3. The argument itself doesn't deserve to be dignified with a response. We saw clearly in 2000 what happened when people decided Al Gore wasn't ideologically pure enough, or too tainted by association with Bill Clinton, or otherwise unworthy of their support. With our nation and our children now doomed to suffer the consequences, that attitude should be regarded as permanently discredited.
I propose, also, that this principle be extended to similar comments in either of these forms:
-- "There's no real difference between Candidate X and [George Bush, John McCain, Britney Spears, etc.]."
-- "I'd rather see [McCain, Satan, Barney, etc.] elected than Candidate X. Maybe then things will get so bad, America will come to its senses." 
Such comments echo exactly the kind of rubbish people were saying in 2000 about Al Gore vis-a-vis George Bush. Look how smart and perceptive they turned out to be! So when we hear this kind of thing in 2008, let's treat it with the icy contempt it deserves.
The second comment, moreover, echoes a favorite slogan of the German Community Party in the last months of 1932: "Nach Hitler, kommen wir." (Roughly: After Hitler, our time will come.) This was the justification for not standing arm-in-arm with the SDP, the bumbling but decent mainstream liberal party of its day. This strategy never works.
So then: How to respond to comments of this kind?
The simplest approach, as always, is to ignore them. But sometimes a more pointed response seems in order -- especially when the poster is being notably idiotic. 
It would be good if we had a name for the kind of wrong-headed argument I'm describing. Then we could dismiss it conveniently by saying, for instance, "Oh yeah, XXXX again, I've never heard that before."
So ... how about ITMT -- as in, I'm Taking My Toys (and going home)?
That's just an idea. I've come to think of it as the Nach Hitler Defense, but I guess that's a bit too recondite. 
Any suggestions are welcome.

... just to calm down his more extreme supporters. Within minutes of Edwards's announcement that he was ending his candidacy -- a model of dignity, class and grace -- I got the first e-mail from my crazy lefty neighbor up the road. Which read in its entirety: "The corporations have won." Is this any kind of fitting end-note to a campaign that succeeded, as Paul Krugman points out in his column today, has completely transformed the Democratic race, and hence the election as a whole? No matter how you feel about Edwards, this isn't the end of the world. Nor is the end of a very important fight. We still have to win the White House. We have to salvage the Supreme Court, the environment, the rule of law, the separation of church and state, and our nation's standing in the world. We've got our little internal squabbles, but we ought to be standing together between now and November, not accusing honorable fellow Democrats of being corporate shills ... and worse. Reading blog posts and user comments at places like Democratic Underground, you get the impression that Edwards supporters are total lunatics. I know this isn't the case -- I was an Edwards supporter from 2004 until some while ago when I got seduced by the excitement surrounding Barack Obama. But let's not go there. The point is, Edwards did NOT quit the race -- as the current most popular thread at DU maintains -- as a result of dark and shadowy forces threatening him and his family with Something Really Bad. He did not give his dropping-out speech "with a gun at his back." He was not "behaving oddly ... not himself ... something in his voice was just wrong..." Look, I've got nothing against paranoiacs, or crazy people in general. Some of my best friends are crazy. A crazy woman raised me. I'm sure you really HAVE had these "strange feelings" before, and they've turned out to be true. I'm sure George Bush saw something really cool in Vladimir Putin's eyes. I'm sure that those people who post in threads titled "Barak HUSSEIN Obama = ANTICRIST" really do sense demonic energy coming off of him whenever she speaks. I'm just a little tired of hearing about it right now. I'm tired of hearing that your guy -- who was a good guy, honest! -- was the ONLY decent and honest candidate in the race, the only one not totally controlled by ... I don't know, whatever vile entity is totally controlling Barack and Hillary, the Corporate Media Illuminati, I guess ... besides maybe Dennis Kucinich, and maybe a couple of other candidates or non-candidates who are safe to support because they (a) are totally unelectable, (b) poll at less than 1 percent, (c) were never in the race to start with, or (d) you've just got a feeling about them -- any or all of which PROVE that they are noble and non-corporate-shill-like and incorruptible and better than MY candidate, who is a total sell-out and a phony, and I'm a gullible fool for supporting him. So please, Mr. Edwards. Do your country one more great service. Endorse ANYBODY who is still in the race. Or just give a speech encouraging your supporters to remain involved in the process and the Democratic Party. Ask them please to stop insulting the two good and smart and honorable people still in contention. And please don't look weird or strange nor robotic or mind-controlled when you make this speech ... because those psychic people at Democratic Underground can tell! And so can the Corporate Media Illuminati, and they'll know you're sending coded signals, and they'll shoot your dog! Thanks.