Several explanations have been offered to account for the anti-Semitism that plagues sections of the Labour Party. More than one of them might be valid. For example, there is within the Labour Party an element characterized by a visceral rejection of all things Western and capitalist. Since Israel is both of these to some extent, they reject what it stands for, and equate pro-Jewish as pro-Israel.
Similarly, support for the aims of the Palestinian peoples is seen as supporting the underdog against the might of an oppressive capitalist predator bent on conquest and oppression. Both elements explain some, but not all, of anti-Semitism within Labour’s ranks. But there is a much larger element, one people find it difficult to discuss. It is the elephant in the room that people pretend is not there, and the name of that elephant is Islam.
In a significant number of parliamentary seats in the UK, a Labour majority depends on a Muslim vote that breaks heavily in its favour. Labour candidates can bid for that vote by expressing anti-Israeli and, by association, anti-Jewish sentiments and prejudices. Jeremy Corbyn and his allies know that a future Labour government can only be achieved with Muslim support, and from this derives their reluctance to condemn the virulent anti-Semitic groups and their spokespersons.
Of course, educated and cultured Muslims are no more prone to bigoted prejudice than their educated and cultured Christian counterparts, but in neither groups do these constitute a majority. Fanaticism is there to be bid for, and so long as Labour bids for it, it will be plagued by anti-Semitism. The elephant is still there, even if everyone agrees not to look at it or to name it.