Godwin’s Law states that “As an online discussion grows longer, the probability of a comparison involving Hitler approaches 1”.  I don’t know whether online discussions have ever been rigorously investigated to determine the truth of the law, but casual observation suggests that it conveys at least a degree of truth.  And I’m not sure that it’s limited to online discussions either. 
There is a corollary to the law which states that “when a Hitler comparison is made, the thread is finished and whoever made the comparison loses whatever debate is in progressâ€�.  The truth of that one is rather less certain in my mind; it more often seems to be the case that those involved simply double down on the positions that they take.  What is perhaps truer is that once a debate reaches that point it changes direction; it loses focus on the original question, whatever that was, and starts to turn around whether the comparison is fair or reasonable instead.  That in turn makes it a poor argument to use in most – perhaps all – circumstances.
Part of the fuss around antisemitism in the Labour Party revolves around whether the Labour Party should accept the internationally-recognised working definition of antisemitism; and one of the disputed clauses at the centre of that debate is that “drawing comparisons of contemporary Israeli policy to that of the Nazisâ€� is, by definition, antisemitism.  I find myself in sympathy with those in Labour who doubt whether this should really count as antisemitism at all.  At a purely rational level, comparing the actions of two different governments in two different epochs and finding some apparent similarities is not at all the same thing as taking a hostile position against a whole people or religion.  I can understand, though, why accusing the descendants of those who most suffered the consequences of a particularly evil regime would be considered by them to be exceptionally insulting, and that alone should make people very wary about either drawing such a comparison or arguing about the detail of the wording.
Coupled with Godwin’s Law, it’s a wholly counter-productive line of argument as well; it diverts the debate away from the reality of the effects of Israeli policy towards the Palestinians and into questions of semantics and the fairness of the comparisons drawn.  The real issue is not whether Israeli policy is or is not similar to that of the Nazis, nor whether drawing such a comparison is or is not antisemitism: it is about the way that the Israeli government is treating Palestinians on a daily basis.  I don’t need comparisons with what someone else did in another place at another time to be able to see brutality and inhumanity in action, and such comparisons don’t add anything to my perceptions of what is happening.  And I really don’t understand why the Labour Party is allowing others to divert it into a row about definitions rather than the substance of the Palestine issue.