Whatever the reasoning that persuaded Theresa May to call an early general election — and whatever the outcome — the voting that will take place on June 8 is likely to mark a watershed moment in the history of British Jewry’s encounter with the British political system.

I started polling Jewish voters in 1974, shortly after the previous year’s Yom Kippur War.

My research suggested that Jewish voters were very willing to punish candidates of whatever party whom they identified as unsympathetic in relation to a range of “Jewish” issues, principally but not only Israel, but that, at the same time, no one political grouping could claim that it was the natural party of choice for the totality of Britain’s Jewish communities.

At Finchley, for example, even at the height of the Thatcher era the Tories were picking up less than two-thirds of the Jewish vote (52 per cent in 1983 and 60 per cent in 1987), while Labour was still polling more than one in five of that vote in this quintessentially Jewish middle-class constituency.

Over the succeeding two decades or so this reservoir of Jewish support for Labour has remained stubbornly resilient: a Survation poll of Anglo-Jewish voting intentions, commissioned by the JC and published in the spring of 2015, showed that, of those Jewish respondents declaring an intention to vote, 22 per cent still said they would vote Labour.

But 69 per cent indicated they would back the Tories.

I shall be very surprised indeed if this pattern is repeated next month, because all the available evidence suggests that the Jewish Labour reservoir is in a state of collapse.

According to a poll commissioned by the JC a year ago, Labour support among Britain’s Jews had then plummeted to a mere 8.5 per cent, while over 38 per cent of respondents gave Labour the highest possible mark — five out of five — for anti-Jewish prejudice among Labour party members and elected representatives.

And, as we know, a great deal of very turbulent water has passed under Labour’s bridge since then: multifarious instances of antisemitic social media postings by elements of the party faithful; the high-profile suspension and then low-profile reinstatement of sundry such miscreants; to say nothing of the Corbyn-factor and especially of the party’s comprehensive mishandling of allegations surrounding that most consummate of socialist Jew-baiters, Red Ken Livingstone.

In the whole of the UK there are not much more than 330,000 Jews, and, that being the case, it might be thought that in the great electoral scheme of things Jewish votes might not count for very much. But the Jewish vote is overwhelmingly urban and heavily concentrated into the bargain. Consequently there are a number of parliamentary seats in which the Jewish vote will be crucial.

At Ilford North, for example, where Labour’s philosemitic Wes Streeting will need every Jewish vote he can get to defend a majority of 589; at Hampstead, where the Labour majority is just over 1,000; and at Harrow West, with a Labour majority of 2,208.

In all these constituencies Jews who have historically voted Labour but who cannot now bring themselves to switch to the Tories will have other options. They might choose to abstain — in itself a powerful electoral weapon.

Or they might vote Liberal Democrat or Ukip. The 2016 Survation poll put Jewish support for the Lib Dems at slightly under four per cent, and for Ukip at about half that proportion.

What might happen to this support on June 8 can only be informed speculation.

But if we consider a constituency as marginal as Ilford North, it is not difficult to imagine that even a trickle of Jewish voters from Labour to Ukip, or more likely in my view to the Lib Dems, could result in Labour losing the seat. The same would be true of Hampstead.

The fate of Jewish support for Ukip is if anything even murkier. It is true that Nigel Farage was and remains sensitively attuned to Jewish concerns. But he is no longer Ukip leader.

The 2017 contest will also decide the fate of a number of MPs — Jewish and non-Jewish — who are broadly sympathetic to Jewish interests.

At the present time, Labour is trailing the Tories in the opinion polls by around 20 points.

If that lead persists, Jewish Labour MPs such as Ivan Lewis (Bury South), Ruth Smeeth (Stoke North) and David Winnick (Walsall North) would all lose their seats, as would Joan Ryan (Enfield North), the current chair of Labour Friends of Israel.

Should Labour lose the 2017 election there will doubtless be calls for Jeremy Corbyn to resign. Will he go quietly? If not, we might witness a further round of blood-letting in Labour’s ranks.

And — perhaps — the virtual destruction of its historic Jewish roots.

Geoffrey Alderman is Michael Gross Professor of Politics and Contemporary History at the University of Buckingham

See all our Election 2017 coverage here