It's always supposed to be deeply suspect to argue ad hominem - playing the man, not the ball - as though it were a fallacy. Well, sometimes it almost is. It's even more frequently very annoying, generally as a tactic being used to avoid the more strenuous business of disputing lucidly the logic of an assertion.
But other times it is an entirely appropriate component of a critique. If, for example, a convicted murderer was to argue that murder doesn't merit a custodial sentence; or a CEO earning £10 million a year was to be found proposing income tax be abolished; then mention of their personal interests in the matters would not be thought out of place. (Which isn't to say their reasoned arguments shouldn't be heard, for what they are worth: the laws of natural justice cut both ways - as the Trans lobby would do well to remember.)
Now here's an anecdote, before I get to the point. A year ago, my local Conservative association invited a well-known political journo on the left of UK politics to address us. He was very interesting, and we had an excellent, open, wide-ranging Q&A after his talk. À propos of somethingorother, he said: do you know, this kind of event couldn't take place within the Labour Party. You people are interested in seeking views and analyses from the other side of the political divide in a spirit of enquiry, and you hear me out courteously. If a mirror-image Labour meeting were to be attended by a right-wing journalist, he wouldn't be allowed to finish before several people would feel obliged to shout him down.
I want to say - because I consider it meaningful - so, what does that tell you about the Labour Party? And the answer, of course, is that it is disproportionately** filled with unpleasant people, and should be discounted accordingly. Nor is there, IMHO, a legitimate counter along the lines of: no, wait a minute, Labour Party members are so down-trodden, nay, struggling for their very survival; you can't expect them to observe genteel pleasantries. In reality, you know full-well the bastards heckling from the back would just be assorted middle-class Trots for whom raw survival has never been any kind of issue.
Which brings us to Labour and anti-semitism. Just consider what's being unearthed in that nasty little organisation, day after day. Enough to damn their enterprise as a whole, that's what. Ad hominem? When so many of the homines (yes, and the feminae) are as deeply unpleasant as that, then yes: the rest of them should be thinking carefully just what it says about the company they keep. The company they propose should be entrusted with political power. It's not fallacious, or even slightly tangential. It's central to the issue.
** Yes, I've met plenty of perfectly reasonable, non-doctrinaire ones too. I feel sorry for them; and in many constituencies I'd say life will soon become very uncomfortable for them indeed.