Britain’s political class has a long and damaging record of not taking politics in continental Europe seriously. A collective insularity tempered only by a worship of all things American ensures that a minor event like this week’s congressional byelection in the suburbs of Atlanta is more likely to register inside the British bubble than, say, the critical contest hotting up for the leadership of Spain’s influential but divided Socialist party.

Yet small buds of change are beginning to open. By far the most important election for Britain right now is the presidential contest taking place across the Channel in France, where the first of two rounds of voting is happening on Sunday. It is hard to remember a French election that has received as much attention in Britain – and for good reason.

This French contest is utterly different from all those that have preceded it since the Fifth Republic was established in 1958. In a contest where turnout will be crucial and almost a third of voters claim to be undecided, Marine Le Pen may emerge victorious as by far the most ultra-right leader of a major western European state since the death of the Spanish fascist leader Francisco Franco more than 40 years ago.

Yet if the polls are right – a big if – Macron will overturn the French political order when the contest concludes

If that were to happen – which many on the ground still suggest it may – the impact on French life and France’s international standing would be devastating. France’s barriers against the world would go up. Parts of the world would treat France as a leper. The EU would face a crisis that would put Brexit in the shade. Nato would be knocked sideways. European nation states would panic that it could happen to them too. Above all, France would turn in against itself. Social and political divisions would deepen in ways that have not been seen since 1945. The stability of the republic would be tested in ways that no western European country has experienced since the Greek civil war 70 years ago.

But while France is a country at a crossroads, the options for change in French politics go far beyond the possibility of a Le Pen victory. It isn’t just the far right that rarely comes within a sniff of the presidency in modern France. It’s also the far left, now in the shape of the uber-Corbynista candidate Jean-Luc Mélenchon, and the centre, in the person of Emmanuel Macron, who remains the favourite to win, provided he makes it through to the second round.

Centre parties and centrist candidacies come in different guises in different political traditions, not just France. Think the Liberal-SDP alliance in 1980s Britain. Think the business-based German Free Democrats. Think Justin Trudeau’s revived centre-left Liberals in Canada, or Spain’s new fourth party, Ciudadanos (Citizens). Macron’s En Marche! movement is different from all of them, because it is so personally focused, completely untested and in many ways peculiar. But centrists all have something important in common – they always struggle to break the familiar though weakened two-party grip of the old centre-right and the old centre-left.

But it can happen, even in France, with its deep-rooted and once-rigid political traditions. As long ago as 1974, Valéry Giscard d’Estaing squeaked through the middle between the official Gaullist nominee Jacques Chaban-Delmas and the socialist-communist unity candidacy of François Mitterrand to become president. Giscard was a centrist of the right, while Macron is a centrist of the left (though he occasionally denies it). But they share many traits – youthful, modernising, liberal on social issues, technocratic, financial backgrounds, Anglophone.

Early on, Macron benefited from the excitement of being new and interesting just when the old parties were looking incapable and tarnished. The uselessness of François Hollande’s Socialist party and the cussedness of the former Gaullists, whose surprise choice of the Catholic free marketeer François Fillon was mired by corruption scandals, allowed Macron to surge.

But is Macron’s remarkable run sustainable in the crucial final days? Le Pen remains strong. Fillon has not collapsed, because there are a lot of deeply Catholic voters, and they have no other candidate. Mélenchon has sucked votes and energy from the official Socialist candidate. As a result, all four main contenders – Le Pen, Macron, Fillon and Mélenchon – go into the final weekend with a chance, probably in that order.

Yet if the polls are right – a big if – Macron will overturn the French political order when the contest concludes in less than three weeks . His problems will be immediate, above all winning a parliamentary majority against a strong Front National. But he will have proved that there is an alternative, a third way, between – as he put it the other day – Thatcher and Trotsky. And he’s right.

It’s beginning to dawn that the result in France affects Britain as well as France itself. The next president will have a big bearing on the UK’s withdrawal from the EU, a significant impact on UK foreign and defence policy thinking, and may even be an important off-stage factor in the general election here in June. Plus, of course, it matters rather a lot to the European continent to which, in spite of Brexit, Britain remains tied by geography, history, trade, culture and sympathy.

A Macron victory would be much the best outcome among the available choices, not just for France but for the EU. And that means, though Theresa May will be loth to admit it, a Macron win would ultimately be good for Britain too. Negotiating with Macron over the next two years will not be easy, because he understandably shares the same priority as Germany and most of the EU27 that the union must be defended and made stronger. On every other count, though, he would be by far the least worst of the options.

A Macron presidency is unlikely to be robust and strong. Many of his reforms will be opposed. But Macron would avoid the inward lurch that Le Pen or Mélenchon would attempt, or the deregulatory anarchy that Fillon might try. A win for Macron would also be a harbinger – inevitably imperfect but real all the same – for the importance of future-focused centrist politics, economic and social reform, and international cooperation.

Merely to write those words is to see instantly that, as the general election gets under way here, there is a large Macron-shaped hole in the centre of British party politics. Corbyn doesn’t want to fill it. May probably can’t. Tim Farron isn’t strong enough. Yet somehow, some time, someone will have to fill it. At least a Macron victory would prove it can be done.