You may know the story of the King of France, woken by the Duc de La Rochefoucauld-Liancourt to be told of the fall of the Bastille. "Is this a revolt?" he asked sleepily. "No, Sire," replied the Duke. "It is a revolution."

The election of Jeremy Corbyn as leader would not be a problem for the Labour Party. It would be a debacle. A catastrophe. A calamity. A disaster.

But the reason why it would be all these things is often misunderstood, even by those who are best at understanding these sorts of things. Alastair Campbell, for instance, wrote a forthright blog calling for anyone apart from Jeremy Corbyn to be elected. Yet his reason for it was that Mr Corbyn would make it hard - almost impossible - for Labour to win the next election.

And that is not the right objection. A party can overcome the selection of a leader with limited electoral appeal. It may be difficult for a while, but things recover. What is hard to cope with is not so much a leader who won't be elected prime minister, but one whom mainstream members of the party cannot honourably vote for themselves.

This is the prospect - let's talk about our own community for a moment - facing those on the centre left in our community.

While Ed Miliband was not wildly popular with most Jews, he took a respectable position on Israel. It wasn't one I shared, and I was heavily critical of it. It even outraged many members of the community, who were bitterly disappointed with his position on the last Gaza action. It was, however, a respectable position, for all its faults.

Jeremy Corbyn is in a different place altogether.

He shares the virulent anti-Zionism of the hard left. One that seeks to make Israel a pariah state. One that treats Israel as if it were the central cause of all foreign affairs problems. One that treats with Hamas and is friendly to Hizbollah.

This takes its place in a foreign policy outlook that would rip Labour from its Atlanticist moorings.

So the question, say, for Ivan Lewis or Luciana Berger, is this: How can they advocate to anyone that this man should be prime minister? And if they can't do that, what do they do? How do you navigate yourself through politics as someone who cannot honourably advise voters to allow the party leader to hold office?

I must emphasise that this is not a taunt. I chose them because they are both people I admire. They are intelligent, brave, progressive politicians who contribute a great deal to public life and have much more still to contribute. I put the questions because I sympathise greatly with the dilemma these very good MPs will - or might - face.

And, of course, they will not be alone. Anybody adhering to an even faintly mainstream position on foreign policy and on Israel will be in the same place. There are tens of thousands of centre-left Jews who will effectively be disenfranchised.

Note that I said that this is a dilemma that they only might face. They aren't facing it yet. The vote hasn't happened.

Now some people think that Jeremy Corbyn is going to do so well that even if he doesn't win, the damage is done. I don't agree. There is a huge gulf between him doing well and him winning. All the difference in the world.

Which is why I am amazed by the placid nature of the reaction within Labour to the prospect of Corbyn winning. As if it was a minor nuisance, a bit of, you know, a pain.

Where are the rallies featuring every person who has ever served in a Labour cabinet or Shadow cabinet? Where is Gordon Brown's national tour? Where is Ed Miliband's personal appeal?

You know how in Dad's Army Corporal Jones keeps saying, "Don't Panic"? Yeah. Well, do panic.