Lots is uncertain in British politics just now, but here's one prediction you can bet on: if Labour goes into the next general election led by Jeremy Corbyn, the party will receive the lowest Jewish vote in its history.

Part of the explanation predates the new leader. Jewish affection for Labour, high during the Blair era, nudged downward under Gordon Brown and fell more steeply under Ed Miliband. But that downward trend will accelerate under Jeremy Corbyn. The most obvious cause is his position on the Middle East. Devoted supporters of Israel will take one look at his long record of vocal opposition to the country and decide he's no friend of theirs.

Still, that won't be the whole story. As we know, it's a minority of Jews in Britain (or the US for that matter) who cast their vote on the basis of policy towards Israel. No, what will push Jewish voters away is something more nebulous. At its simplest, it's the company the new leader keeps. Perhaps British Jews could overlook that one of his closest backers is Ken Livingstone or that George Galloway has vowed to rejoin a Corbyn-led Labour party - two men about whom the mainstream Jewish community made up its collective mind long ago.

Harder to dispel will be unease that the new leader of the opposition hosted representatives of Hamas and Hizbollah and greeted both as "friends". That he wrote a letter defending the notorious Rev Stephen Sizer, disciplined by the Church for spreading "clearly antisemitic material" online, including the claim that 9/11 was a Jewish conspiracy. That the self-described Holocaust denier Paul Eisen describes Corbyn as a constant and loyal ally, one who opened his cheque-book to back Eisen's Deir Yassin Remembered group. (A quick look at Eisen's website confirms its toxicity: there are musings on the supposed physical ugliness of ultra-Orthodox Jews and on whether the key political dividing line is not between left and right but between "Jew and gentile".)

Corbyn's backers have two lines of defence. The first is to brand his accusers McCarthyites, smearing him through guilt by association: surely a busy, campaigning MP cannot be held responsible for all the people he happens to have sat next to at public events. But Corbyn didn't just end up on a platform with these characters. Often he was hosting these meetings himself, inviting the guests. The letter defending Sizer was no accident: the words were Corbyn's and carefully chosen. As for Eisen, there has been no denial that Corbyn wrote a cheque; and there are photographs to prove he kept turning up at Eisen's events.

The party will receive the lowest Jewish vote in its history

The second defence says yes, Corbyn met all kinds of extremists but he did so only in the cause of Middle East peace - out of the sincere belief that any resolution will depend on the participation of hardliners as well as moderates. That sounds like a legitimate line of argument - and for a diplomat or peace negotiator, it is. Except that's not the business Corbyn was in. The purpose of those Westminster meetings was not conducting diplomacy, but expressing solidarity. If Corbyn were truly a peacemaker, anxious to bring all sides into the process, he'd have regularly invited the militant Jewish settlers of the West Bank to meet him in the Commons - always greeting them as "friends." Yet, funnily enough, that never seems to have happened.

No one is suggesting Jeremy Corbyn is an antisemite. But his challenge now is to demonstrate that he's not alarmingly comfortable with those who are.

Jonathan Freedland is a columnist for the Guardian