ALONG with Sherlock Holmes and the rules of football, one of the great legacies of Victorian Britain is the Westminster parliamentary system. If voters want their voices to count, they have to choose between two large, boring parties. Excitable fringe groupings and a smallish third party, spread thinly across the country, struggle to make their mark. In return for putting up with such rank unfairness, Britons are promised strong, stable governments that can get things done. Plenty of countries have thought that trade-off worth copying.

But with three months to go until a general election, the mechanism is broken. In 1951 the Conservative and Labour parties together scooped 97% of the vote; in May, opinion polls suggest, they will each win barely a third. Membership of the Tory party has fallen from 3m in the 1950s to about 150,000. Labour, which used to rule Scotland, could be reduced to a handful of seats there. Support for the Liberal Democrats, tarnished by coalition government, has collapsed. Almost all the running has been made by three insurgents: the Scottish National Party (SNP), which wants Scotland out of Britain; the UK Independence Party (UKIP), which wants Britain out of Europe; and the Green Party, which wants “hyper-capitalism” out of both Britain and Europe. It is the biggest shake-up since the early 20th century, when Labour displaced the Liberals.

Political insurgency—from Syriza in Greece to the Tea Party in America—is a feature of many Western democracies. But it will hit Britain especially hard. Unaccustomed and ill-adapted to multi-party politics, Britain is more likely to get weak, unstable governments. That will only fuel the dissatisfaction with career politicians in the main parties. And if the parliamentary system comes to be seen as both unfair and ineffectual, then it is in for a crisis of legitimacy.

Grief in the land of Gladstone

Some within the Tories and Labour hope that Britain’s political duopoly will re-establish itself as memories of the financial crisis fade or the first-past-the-post system bulldozes the smaller parties. That is unlikely to happen soon, because long-run forces are at play (see article). As the British class system has given way to a mish-mash of socio-economic groupings, tribal loyalties to Labour and the Conservatives have evaporated. Voters no longer belong to the left or the right. They have become political consumers who shop around. More recently, proud Scottish, nostalgic English or anxious green identities have counted for more than whether someone works on the factory floor or oversees those who do.

Waiting for the rebirth of the two-party system would also be harmful. Although the insurgents promise voters choice, the fracturing of politics is dangerous. One reason is that they have no agenda for governing. Although the Greens, the SNP and UKIP promise an end to politics as usual, all they put forward in its place is an escape from hard decisions. None has a plausible economic plan. Nor are they likely to be forced into coming up with one by the sobering experience of a big role in government. Having seen the Lib Dems’ collapse, UKIP and the Greens have ruled out joining a coalition. The insurgents can spoil politics, but without being able to break the grip of the two big parties. They are revolutionaries without guns.

The other reason to worry is that the system’s unfairness could become destabilising. A familiar injustice is that a disproportionate share of MPs will continue to be Tory or Labour—in May their two-thirds of the vote could win 90% of the seats. But layered upon that is a second, new distortion. Because their support is thinly spread, the Green Party and UKIP could together win a quarter of the votes, but only six or seven seats out of 650 in the Commons. In contrast, the SNP, is on track to win just 4% of the vote—putting it in sixth place—yet could scoop 40 seats to become the third-largest party.

If Labour and the SNP form a coalition, policies carried by SNP votes against the wishes of a majority of the electorate will seem especially illegitimate. Imagine, for example, a reversal of welfare cuts or a promise to remove Britain’s nuclear submarines from Scottish waters. The SNP might also demand a second independence referendum as its price for coalition.

All coherence gone

The political process needs to adapt. If no party wins a majority in a general election, a few weeks should be set aside for them to thrash out a deal, as happens in much of Europe. MPs from the coalition parties should then vote on their agreement, so that backbenchers cannot argue (as some Tories now do) that they were bounced into a coalition against their will. The Fixed-Term Parliaments Act, which encourages weak governments to limp along, should go.

More fundamentally, mainstream politicians must deal with the fear and resentment that is driving voters away. Trying to outdo the insurgents on immigration and fracking is harmful and doomed to failure. Rather than ape the insurgents’ policies, mainstream parties should copy their methods by becoming looser and better able to rally ad-hoc groups to worthwhile causes such as pensions reform and house-building. Rather than look to career politicians, they could recruit and promote outsiders. Rather than peddle fantasy, they should be honest, telling voters frankly that the forces of technological change and globalisation are unstoppable and inescapable—and that years of difficult reform lie ahead.

But constitutional change will be needed, too. This newspaper has long argued for proportional representation (PR)—a version that combines first-past-the-post, to preserve a direct, singular link between MPs and their constituents (a rare aspect of British politics that is popular and works), with a top-up based on the national share of the vote. A dose of PR would not only deal with the system’s unfairness, it would also counteract the unhealthy regional polarisation of seats in which Scotland belongs to the SNP, northern England to Labour and the south to the Tories.

The battle over the details will take shape after this election, and future ones. Meanwhile Britain’s voters should strap themselves in. It is going to be a bumpy ride.