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.