In politics, being popular is good but populism is something different, and it threatens our democracy. Left and right wing populism have a lot in common and they are both part of the same danger. Extremist ideas like racism, xenophobia and antisemitism have returned to the mainstream.

Populism splits everything into good and bad: ‘the people’ and ‘enemies of the people’. ‘The people’ is an idea, the opposite of the material diversity of flesh and blood human beings.

Politics is about representing people, with different tastes, opinions, values and dreams; people from different classes, genders and origins; with different interests, talents, and problems. Democracy as a whole, including Parliament, the rule of law, civil society organisations, international co-operation, democratic freedoms and cultures, is how we negotiate our differences and find ways of living together.

Our democracy is not perfect, but it is a huge achievement nevertheless and we should build on it, not devalue it.

But ‘the people’, as conceived by populism, has one interest, one appetite and one resentful fury. Because ‘the people’ is really an abstract idea, it cannot articulate anything; it needs a leader to understand its eternal soul and to speak with its voice.

Populism begins with the feeling that nothing that exists now is of any value and it is driven by sentimental nostalgia for a past which never really existed.

The populisms of the left and of the right agree that the real enemy is the ‘liberal establishment’ which is portrayed as only cunning and selfish. It is accused of pretending that society is based on freedom and democracy but secretly running everything according to its own secret self-interest.

News, according to the populists, is lies, concocted so that we will vote how ‘the establishment’ wants us to. Politics, for the populists, is a racket, in which a ‘political class’ only pretends to look out for the good of the country.

Populism treats science and expertise as mortally corrupted by power. Even the principle of human equality, the populists think, is sneakily transformed by those with power into a way of keeping us unequal.

The referendum of 2016 made one day holy. An election gives representation even to the losers but a referendum sets up a new timeless truth and it prevents ongoing debate. The demagogues took ownership of the referendum result and they appropriated the right to interpret what it meant.

Populism is utopian. Johnson does not promise to make Britain a little better but to make it the best country in the world. Corbyn does not promise to make Britain a little fairer but to abolish injustice. Populism has contempt for the rational policies that might improve things. Instead, it thunders that it will rip everything down rebuild a new world out of the ashes.

It is easier to destroy than to create. Populism will never deliver what it promises. That is why the idea of the ‘enemy of the people’ is so important. Failure will be blamed on the cosmopolitans, the liberal elite, the people of nowhere, the people ‘without roots in the community’, the people with no culture, those who are only interested in money and in ‘the one per cent’.

Antisemitism has evolved through many distinct species to become a ready-made emotional framework for imagining all of these enemies in one potent vision of evil.

We have seen antisemitism emerge into the mainstream left. In its appearance as ‘criticism of Israel’ it is still plausibly, to some, deniable; yet antisemitism is ‘educating’ a whole layer of left activists that ‘Zionism’ and ‘right wing Jews’ stand between ‘us’ and socialism.

Right populism is also structurally similar to antisemitism and it is creating fertile conditions for the emergence of its own antisemitic movement. Brexit can only be ‘betrayed’ since any Brexit which may be delivered would fall short of the fantasy.

It will lead to crisis, people losing their jobs and to a new and deeper ‘austerity’. Right populists will look for somebody to blame. They will find ‘enemies of the people’ and some of them will picture the enemy with a big nose and a grasping cunning.

Both of the big parties have been killed, hollowed out, and animated by hostile populist factions. This is not a crisis of the left or the right but of both. There could be no Corbyn without Brexit and there could be no Johnson without Corbyn.

Voters will think of supporting one populist leader for fear of the other but the price of that would be to strengthen the populist culture as a whole.

The alternative is to vote for anti-populist candidates, to maximise opposition to populism in Parliament, and to build a movement tough enough and smart enough to defend liberal democracy.