Let me introduce you to the Blair paradox.

Tony Blair — a man for whom I have tremendous respect — has been arguing that, as Theresa May is going to win, what we really need is a strong opposition. And he has got a point, hasn’t he?

But here’s the problem. There can only be a strong opposition if Jeremy Corbyn is reasonably successful. If Jeremy Corbyn is reasonably successful then Corbyn and/or the Corbynbistas can stay. And if Corbyn and/or the Corbynistas stay we will have a weak opposition. The Blair paradox.

I have a lot of friends who vote Labour and I understand their dilemma. They have supported Labour all their lives and they don’t want to abandon their party to Jeremy Corbyn. Unfortunately, not abandoning the party to Mr Corbyn means supporting the party while he leads it.

Despite the acuteness of the dilemma, this is unconscionable.

I realise that I am a Conservative peer and this point concerns party politics. But, still. Forgive me for this is a point I feel I must make as a Jew.

If Jeremy Corbyn and his followers do not suffer a gigantic defeat in this election, it will be an utter, complete, ghastly disaster for Jews.

It will mean that despite all that has happened in the past two years, all his supporters have said about Jews, people — even Jews, for goodness sake — can still support him. What’s a little antisemitism between friends?

Indeed, respected community leaders are about to set out trying to persuade other Jews that it is all right voting for a party whose leader calls Hamas friends.

Why? “Because he is going to lose”.

Well, first off, let me make a simple point. He won’t lose if people vote for him. Asking to support his party on the basis that it won’t win is saying: “Don’t worry, you can do something completely wrong, indeed unconscionable, because no one else is going to.”

“Vote Labour — it won’t make any difference.”

It is also the case that candidates and canvassers are asking people to vote in as a government people who they think won’t form a government and — worse — people who shouldn’t form a government.

But here’s the other point. Jeremy Corbyn mustn’t just lose. He must be crushed electorally. It must be impossible for his supporters to say that it wasn’t too bad and they should have another go.

He mustn’t be able to form a strong opposition.

The leadership that didn’t expel Ken Livingstone and allowed the rise of Jew hatred among its supporters on social media can’t just do moderately badly.

I’m sorry to bring this down to brass tacks, but how would the Jewish community look if, after all that had happened, Labour still won in Finchley and Golders Green? Within a few months Ken Livingstone is set to be back in that party.

I’m not suggesting that people who aren’t Conservative, vote Tory. This isn’t the place for that. I am just saying that there are plenty of alternatives.

I was delighted that Tim Farron acted to remove David Ward as a Lib-Dem candidate because it means centre-left friends have a mainstream progressive option.

I am also not suggesting that people abandon their attachment to Labour for ever.

Labour is a great party with a proud history and many great achievements. Labour has been a wonderful friend to the Jews and home to many of our greatest leaders. It has been an ally to Israel and as brave as any party in its defence. There will be a time for people to go back to Labour.

But that time isn’t now.

I realise this is very, very difficult. But let me try this point. This is an opportunity to defeat Corbynism heavily. If it isn’t taken, it won’t be Tories like me that suffer. I can go on opposing what Corbynistas stands for quite happily.

The problem will be for my Labour friends. Fail to act now and you will never get your party back. If his behaviour has few consequences and you backed his party anyway, why should Labour return to you? You’ll be with them whatever.

So it needs bravery now to secure the long term future of Jews on the centre left. Maybe I’m not the right person to give this advice. I can see that. But forget it’s me, I am right aren’t I ?

Daniel Finkelstein is associate editor of The Times

See all our Election 2017 coverage here