IN HIS dystopian novel of 1935, “It Can’t Happen Here”, Sinclair Lewis described the rise of an American Caesar, Berzelius “Buzz” Windrip. Buzz easily defeats Franklin Roosevelt for the presidency by promising to make America great again. He then sets about destroying the country’s system of checks and balances, by fomenting fear and unleashing activists, while sensible Americans comfort themselves with the belief that their country is immune to authoritarian takeover.

Donald Trump’s election has propelled Lewis’s novel back onto the bestseller list and provoked a lively debate on the question of “Can it Happen Here?”, the title of a new book edited by Cass Sunstein, a Harvard professor and former adviser to Barack Obama. It is time for Britain to engage in a similar debate. The British are even more confident than the Americans about their immunity to extremism. Britain hasn’t had a violent revolution since 1640-60. Rather than rallying to Oswald Mosley’s fascists in the 1930s, the British treated them as figures of fun—black shorts rather than blackshirts, in P.G. Wodehouse’s satire. But the next five years could test Britain’s immune system to the limits.

One threat to the liberal order comes from Jeremy Corbyn’s Labour Party. Mr Corbyn is a classic left-wing populist, convinced that life is a never-ending struggle between the virtuous masses and the wicked elites. Some of his main advisers are Marxists who regard political institutions as instruments of class power. Mr Corbyn became leader by bypassing Labour MPs and appealing to party activists. He has warned right-wing newspapers that “change is coming”. John McDonnell, the shadow chancellor, called for a million people to take to the streets to protest against the result of last year’s general election. High among Labour’s priorities is repealing legislation that prevents co-ordinated strikes.

A second threat comes from the incendiary right. Brexiteers invoke the “will of the people” to suggest that anything but their own maximalist interpretation of Brexit is illegitimate. Theresa May has revived an ugly 1930s trope about “citizens of nowhere”. The Daily Mail has described judges as “enemies of the people”. In a recent tweet Nadine Dorries, a Tory MP, labelled Sir John Major, the leader of her party in 1990-97, a “traitor”.

Such extremism is self-reinforcing. Angry people feed on each other’s anger, sensible people retreat into private life, and institutions are weakened in the tussle. This is already beginning to happen. Political activists are increasingly willing to bully their way to power. MPs—particularly moderate ones—report an upsurge in threats and smears. Intimidation is becoming routine on university campuses. On March 5th a group of masked protesters invaded and disrupted a talk at King’s College, London, put on by the college libertarian society.

The cycle of extremism could get worse very quickly. Imagine that Mr Corbyn wins the 2022 election—the most likely outcome—and starts putting into practice his policy of encouraging the democracy of the street as well as the debating chamber. The Conservative Party might well respond to this by embracing British nationalism and unleashing its own street warriors. An epidemic of strikes and demonstrations could have the British public crying for a blond beast to restore order.

Britain has weak formal defences against authoritarian populism. It is one of the few countries, along with New Zealand and Israel, that doesn’t have a codified constitution to protect basic rights. Since Britain joined the European Economic Community in 1973, European law has filled that void. But, as Vernon Bogdanor of King’s College points out in a new pamphlet, Brexit will remove those protections. Britain is the only advanced country that is weakening rather than strengthening constraints on legislative power (Israel, for example, is at work on a codified constitution). It is doing so just as illiberal populism is on the rise.

At the same time Britain is vulnerable to global shocks. It has an open economy with a huge financial-services industry, a flexible labour market that acts as a magnet to foreign workers, and a capital city that houses some of the world’s richest people. This economy is about to be subjected to one big shock, in the form of Brexit, and may well receive a second, in the form of Prime Minister Corbyn. All respectable forecasters agree that leaving the EU will reduce the rate of economic growth at least in the short term. The question is by how much. The combination of Brexit and a far-left Labour government could lead to a flight of capital, as investors seek safer havens and more predictable political regimes.

Fasten your seat belts

The biggest threat to British institutions, however, comes from a growing sense that democracy has let people down. Stephen Holmes of New York University points out that liberal democracy is a “time-tested system for managing political disappointment”—once you’ve lost patience with the existing elite you can vote them out. But disappointment is surging, at a time when democracy’s ability to manage disappointment is declining. Young people have been encouraged by policymakers to borrow to go to university, and schooled by internet firms that satisfaction is only a click away. But stagnant wages and rising house prices mean that even graduates can’t live as well as their parents. And democracy, by its nature, cannot offer the instant satisfaction of Amazon. Yascha Mounk of Harvard points to disturbing polls. The proportion of Britons who support a “strongman leader” has increased from 25% in 1999 to 50%. The under-25s are much more critical of democracy than people of the same age were two decades ago.

It is too early to head for the exits. Mr Corbyn’s bark may be worse than his bite. Brexit may be manageable. And Britain’s informal defences against extremism may prove strong. Only this week Jacob Rees-Mogg, one of the fiercest Brexiteers, gave a brilliant defence of free speech in Parliament. But anyone who doesn’t know where the exits are is a fool.