Jeremy Corbyn has presided over Labour's worst week ever. Until next week

The Labour Party has just had its worst week ever. Worst week ever, that is, since last week. And worst week ever, probably, until next week.

A crisis reveals all your weaknesses. Disrupting the normal run of events, it demands quick responses which themselves, in turn, rely on sound judgment. Leadership is tested – and the public see in an instant whether you are up to it.

The Paris attacks were France's 9/11. A murderous assault of that nature demands a response that rises to match the scale and horror of what happened. Parisians rose to it showing a fragile and tentative bravery that demonstrated that to the motto "liberte, egalite, fraternite" had been added a new word "solidarite".

Francois Hollande rose to it too with his political, policing and military response as to this "act of war". England fans did too with their moving rendition of "Le Marseillaise".

And Jeremy Corbyn? He showed once more that he never misses an opportunity to miss an opportunity.

Asked about whether the police and security forces should have a shoot-to-kill policy, Corbyn struck precisely the wrong note. He could have supported existing policy in use of lethal force. He could have said he trusted the police in these difficult times. Instead he hesitated – clearly realising that his actual views were deeply unpopular ones – and half-heartedly described shoot to kill as "dangerous and counterproductive". It was as bloodless a statement as has ever been made after a massacre.

You would describe it as the reaction of a desiccated calculating machine – if that were not unfair to calculating machines. Where was the passion? The desire to protect fellow Londoners, fellow citizens. Absent.

Corbyn was forced to reverse himself after a disastrous meeting of the Parliamentary Labour Party where MPs treated him like a supply teacher in a rough secondary modern – barracking him and talking over him. But the damage was done. Not simply because the original disastrous formulation was out there.

But also because Corbyn's true feelings had been expressed. For him shoot-to-kill takes him straight back to the time of the IRA and their terrorism – which Jeremy supported as an "armed struggle". While support for the IRA is in the past, the attitude towards terrorists is very much of the moment.

Ironically, the one Left-winger in the UK Corbyn wants to exclude from the Labour Party – George Galloway – gave a master class on the BBC in what to say about terrorism and how to say it. He first put shoot to kill in context:

"Well you know me, so you would never consider me as a liberal. And I think that on these matters one has to be iron and steel hard.

"One has to say that if anyone comes here with guns and bombs, our police will shoot them down and stop them. There's no room for equivocation about that at all.

"Of course, a shoot to kill policy in general is a bad idea ... apart from being wrong, they don't work, they make more terrorists."

But then he went even harder:

"What the Labour leadership should do is be absolutely clear that when it comes to the safety and the defence of our people in our own island, on our own streets, then the police will have the full backing of the political leadership to gun them down if necessary.

"I'd shoot them myself with my own hand. I'd pull the trigger myself. I'd be happy to see them all dead in the street.

"These people are just about the most horrific group of people I have ever seen in my long political life and I'm damned sure I'm not going to sit idly by while they are running around up and down Piccadilly with guns and bombs."

As eloquent an expression of muscular, populist, antiterrorist leftism as you are likely to hear. No wonder Michael Portillo's response was to ask if Galloway was available to lead the Labour Party. And no wonder Corbyn is committed to keeping him out.

Also, what a contrast not just to Corbyn, but to John McDonnell who had his own special role in Labour's worst week ever.

The Sun unearthed the principles and demands of the Socialist Campaign for a Labour a Victory (SCLV). Among these were a call for MI5 to be disbanded and the police to be disarmed. Standard for the far Left, but of great interest because they were signed by both Corbyn's policy chief Andrew Fisher and the Shadow Chancellor. McDonnell's defence started with a schoolboy error. He made an easily disprovable denial. He hadn't signed the letter.

The problem, of course, is that his name was on the letter. When this was pointed out, he clarified that he had signed up for the principles rather than the demands. So, we had a Shadow Chancellor claiming he signs things he hasn't read – not the best approach to bring to the country's finances. But that is an insignificant error compared to the denial that he didn't know what the demand were since there was a photo of him proudly posing with the finished and printed letter.

Rule one of denials is to have a defensible truth – one that cannot be disproved. That means you can adhere to rules two and three – you only have one chance to tell your story and you should never change your story. McDonnell must have remembered posing with the SCLV demands.

After all, at the time this was not merely one amongst many events in his busy media schedule – the ultra-Left press was his media schedule. It gets worse, there are minutes showing McDonnell was at he meeting when the demand to disband MI5. Or rather, it doesn't get worse – the dogs in the street know that John McDonnell has never met an ultra-Left demand he didn't like. The story of the Socialist Campaign for Labour Victory confirms rather than reveals.

Sandwiched between these two stories was the affair of Ken Livingstone's bungled appointment to co-chair the defence review. Set aside the breach of party rules, and the insult to Maria Eagle, this was an appointment that Ken proved himself unfit to serve. Indeed his spectacularly offensive response to MP Kevan Jones questioning his expertise was one of the worst Labour gaffes under no pressure since Corbyn's election – and there is fierce competition on that front. Apart from the insult, the only credentials Ken claimed were local government expertise – an arena not noted for either geopolitics or grand strategy.

Anyway, judging by how highly emotional Ken was all day, I would want him kept a long way from any nuclear button! The appointment of Livingstone was bad enough. The failure to apologise worse. But the nadir was surely when Corbyn didn't sack Livingstone – that, like Ken, was indefensible.

Can it get worse?

It surely can. Labour has many worst weeks ever in its near future. Perhaps even weekly. But the really big cloud on the horizon is the Oldham West by-election. Labour's white working-class vote has haemorrhaged already. The question is how long can Labour hold on to the Muslim vote.

As George Galloway points out, the vast majority of victims of Isil are Muslims – the antics of Corbyn and McDonnell have no more attraction for British Muslims than any other voters in Britain. Oldham West is, by any measure, one of the safest Labour seats in the country. Do not be surprised by an earthquake there.