Milliband, the leader before Corbyn, was also undermined by the zionists in the party because of his support for Palestine but couldn't use the "antisemite" tag because he's Jewish. Instead they labeled him "toxic" & said "Jewish donors were leaving the party in droves"

https://www.independent.co.uk/news/uk/politics/labour-funding-crisis-jewish-donors-drop-toxic-ed-miliband-9849299.html

So the answer is that unless labour party leaders swear alliegence to Israel, they will face the wrath of vicious and toxic zionists.