Interesting, Heric. The basic definition is very similar in the UK - matter is defamatory (subject to a few twiddles) if it reduces the subject in the eyes of right-thinking (see Byrne-v-Dean, accusation of providing information to police not defamatory because right thinking members of society would favour such action) members of society generally or tends to cause them to be shunned or avoided. This is very similar to the definition you cite, but not the same. It was the difference that made me wonder whether someone had looked up a bit and then filtered their own utterance through the American political perspective...   Then you enter the murky realms of the defences such as (possibly, in this case) qualified privilege or (improbably) justification or fair comment.

Trade libel may add causes of action where there is an attribution of incompetence in a trade profession or calling.

I find it almost inconceivable that an accusation of anti-semitism is not defamatory, particularly of a person holding some public office. An accusation of bigotry would be close. Once upon a time I used to advise a number of UK TV companies on avoiding libel litigation, and I would have had no hesitation whatsoever in telling them not to broadcast an accusation of antisemitism unless they had several smoking guns to back it up.

Nonetheless, it tends to appear that the claim so far has been an exercise in evidence gathering, so the principal issue is whether Google may be obliged to reveal the identity of the blogger so that the blogger may be sued (which latter action might succeed or fail) so the current analogy is rather that of the RIAA seeking identities of file sharers so that they may be sued for copyright infringment (which latter actions might succeed or fail).