Even in the darkest moments of his ill-fated leadership, it’s difficult to imagine Iain Duncan Smith being treated nearly so brutally by the Tory left as was Jeremy Corbyn by his MPs on Tuesday night.

According to the Mail on Sunday the Labour leader attended a karaoke party which he “hoped… would restore harmony with backbench critics”. Instead several took the opportunity to humiliate him (and don’t seem to have managed to harmonise in that, if the video is any indication).

Mike Gapes sang ‘Back to the USSR’ whilst Mary Glindon opted for ‘Like a Virgin’ – a dig at Corbyn’s backfired publicity stunt about failing to find a train seat.

Perhaps most tellingly “dozens” of MPs joined Ruth Smeeth – who received anti-Semitic abuse at the launch of Corbyn’s anti-Semitism report – for a rousing rendition of ‘Things Can Only Get Better’, Tony Blair’s 1997 election anthem. Long before the end of the evening the leader, John McDonnell, and other allies had walked out – or “exited amidst jeers”, as the Mail on Sunday has it – but apparently Tom Watson stayed on.

It’s not exactly a secret that Labour MPs are sick to the back teeth of Corbyn by now, but there’s still something extraordinary about this sort of conduct.

It shatters any veneer of party unity, such as might be sought by those who want to avoid Corbyn scapegoating the moderates when he fails, but isn’t any sort of organised, effective rebellion. It makes the party look simply unprofessional, and suggests not so much desperation as resignation.

Labour is in dire straits. Between Brexit and Scottish independence politics is reorienting around constitutional issues on which the party has no distinctive position, and all available evidence suggests it is marching towards a truly frightening electoral reckoning. A lot of these problems predate Corbyn. We wrote in May 2015 that:

“It is hard to imagine how a single Labour candidate or strategy could at once maintain their position in London, advance against the Conservatives in the South, shore up the North and Wales, and start to fight back in Scotland.”

Nonetheless, it can certainly be argued that by failing to properly mobilise Labour behind Remain, and thus possibly handing Leave the Brexit referendum, the hapless Labour leader is to blame for transforming his party’s chronic, long-term challenges into an acute crisis which he’s incapable of meeting.

It was that failure in the referendum which finally pushed furious Labour MPs to try to unseat him this summer, an ill-planned, furious spasm of an insurrection which culminated in Owen Smith’s hopeless challenge and a renewed mandate for Corbyn.

Hence, perhaps, the willingness to humiliate their leader exhibited by so many Labour MPs on Tuesday night. They know what a horrible position their party is in, but they can’t do anything about it whilst Corbyn is in post – and they can’t do anything about Corbyn, save to assail him with old songs.