Sargent: Portraits of Artists and Friends National Portrait Gallery , until 25 May Sir Jacob Epstein: Babies and Bloomsbury Foundling Museum , until 10 May

Jeremy Corbyn's eye-swivelling about-face on the EU - he once wanted to leave, now he wants to stay - has become a source of mirth for Eurosceptics and a sign of hope for Europhiles. To the anti-EU lobby, the fact that Corbyn voted against staying in the common market in the 1975 referendum and against EU treaties as an MP, yet now wants us all to vote to stay in, shows what a slippery character he is. For the Brussels-loving brigade it confirms that even the most heathen of EU haters can see the light. The 'sinner who repents' - actual words used in the Guardian's editorial on the newly pro-EU Corbyn - could be a useful tool for swinging the vote, apparently.

Yet even as we muse over, or mock, Corbyn's tectonic shift, we mustn't overlook the bigger story here. Which is the strange death of left-wing Euroscepticism. It isn't only Corbyn who's done a 180-degree turn from viewing the EU as a democracy-stifling monolith we should run a million miles from to saying 'Oh, what the hell, let's stay in, I'm sure we can make it work'. This sad, cravenly cautious adoption of a better-the-devil-we-know approach to Brussels has happened across a left that was once more agitated by the EU than the right was.

So Corbyn has previously denounced the EU's 'gross abuse of human rights and natural resources' and said its aim has always been to 'create a huge free-market Europe'. Tony Benn, hero of Corbynistas, and much respected beyond that scruffy circle, consistently took an anti-EU line. A couple of years before he died he gave a talk at the Oxford Union in which he said the EU stifles parliamentary democracy, and by extension weakens the clout of the people. 'It's not that I'm hostile to foreigners, but I'm in favour of democracy', he said, to the bamboozlement of the gathered Oxford leftists, for whom loving the EU is as natural (and unquestioned) as breathing air.

In 1983, Labour became the only major party (unless you count Ukip) to put getting out of Brussels into its actual manifesto. I know that manifesto was the 'longest suicide note in history', but surely those of us who are Eurosceptic can agree that legging it from the then EEC was its one sensible proposal? As the Beeb recently said, 'To go back to the 1983 manifesto is to be reminded that the Labour position on Europe has been utterly reversed'. Many trade unions have also 'utterly reversed', having once been suspicious of Brussels and now being more suspicious of those who oppose Brussels.

The left's weird cosying up to a vast institution it once agitated against is best captured in the figure of Yanis Varoufakis. This is a man who was helping to run Greece when the EU was treating the country with imperious contempt, effectively blackmailing its elected leaders to override the stupid public's wishes and enforce austerity. And yet how has Varoufakis been titillating lefties in Britain in recent weeks? By saying we should stay in the EU. The EU might be 'disgraceful' and a 'disintegrating mess', but we should stick with it, he says. What's up with these people, seriously? I normally hate the use of the term 'syndrome' to explain people's political behaviour, but I'll be damned if Varoufakis's outlook doesn't sound a little like Battered Wife Syndrome. 'They treat us terribly, but let's stay.'

How did being a decent leftie come to mean sneering at Eurosceptics where it used to mean being one? Partly it's about the politics of fear. Lonely, lost leftists, now more likely to fear ordinary people and their passions than want to stir them up, are desperate to dodge any kind of political and institutional upset, because who knows what would happen as a result. And so they cling to Brussels, viewing it as the political glue of a continent they fear would otherwise be rent asunder by conflict and prejudice and stupidity. There's also a strong whiff of virtue signalling. Being pro-EU is sometimes a pretty thoughtless stance, designed to show you're Good and Cosmopolitan and not like those fat blokes with tattoos who pronounce England with three syllables. It's like wearing a ribbon.

But more fundamentally, the strange death of left-wing Euroscepticism speaks to the left's abandonment of the ideal of democracy. The further removed the left becomes from everyday people, the more it views the public as an obese, probably racist blob to be re-educated rather than as political citizens to be engaged. The left's turn from hating the EU to at least wanting to stick with it is directly proportionate to its loss of faith in the masses. Democracy is no longer seen as a tool of progressive change. Lefties now trust EU suits more than they do the loud, odd locals of their own towns.

To my left-wing mind, lefties should be at the very front of the fight to leave the EU. Our movement - such as it exists anymore - claims its heritage from the Levellers, the Chartists and others who insisted on, and died for, the fundamental right of a people to consent to the institutions that govern them. And yet we can't muster the energy and the daring and the ideas to argue against a Brussels machine that is explicitly designed to do politics over the heads of the governed? For shame. Not content with having abandoned freedom of speech (in favour of demanding restraints on hate speech), and universalism (in favour of disappearing up the fundament of a poisonous, separatist politics of identity), now the left is jettisoning democracy too. Which of its values is it going to keep? Any? God, it's mortifying being left-wing in the 21st century.

Leftists who say we should stay in the EU bring to mind those awful radicals who stayed in the Communist Party of Great Britain even after the Soviets crushed the Hungarian Revolution in 1956. 'Yeah, that was bad, but Soviet Communism is still a good idea,' they said. Now, even after the EU has imposed technocratic governments in Italy and Greece, treated the Irish government as a bunch of thick Paddies, libelled the French and the Dutch electorates for voting against EU treaties, and given rise to a Byzantine system of governance largely beyond the reach of Euro-plebs, they say: 'Yeah, that's all bad, but the EU is still a good idea.' My question is this: how many crimes against liberty and democracy must be committed in the name of a 'good idea' before that idea stops being good?