You have got to hand it to Charles de Gaulle. The electoral system he created for France’s Fifth Republic has stood the test of time. More than six decades on, this week’s re-election of De Gaulle’s latest successor, Emmanuel Macron, is a reminder that the particularities of electoral systems can set the terms of a nation’s politics more lastingly than we sometimes allow. There’s a message for Britain there too, but we will come on to that. De Gaulle’s constitution, constructed between 1958 and 1962, aimed at two goals in particular. The first was to empower De Gaulle and his successors to govern as executive presidents, embodying what the general’s biographer Julian Jackson called “a certain idea of France”. The second aim was to keep the French left, and the Communist party in particular, out of power for as long as possible. The system worked well for more than two decades, especially while the imperiously charismatic De Gaulle was still alive, before François Mitterrand deftly reshaped the left while the communists slid into history. Since Mitterrand’s era, however, both the old left and the old right have fragmented into multiple smaller parties. Yet De Gaulle’s electoral system was still robust enough this week to dispatch France’s extreme right very effectively, and for the third time since the millennium. Chapeau to the general for that. Much of the assessment of the 2022 election has been rather grudging about Macron’s victory. It has concentrated on his personal aloofness, on the economic and social problems ahead, on the decline in his majority since 2017, and on the 41.5% share achieved by Marine Le Pen in the second round. These are important things. The national assembly elections in June will be much messier. But the judgments on the presidential contest underplay both the role of the Gaullist constitution itself and of Macron’s success in mastering it, something which eluded his two predecessors, Nicolas Sarkozy and François Hollande. Macron’s re-election shows that De Gaulle’s system still has gas in the tank. His 28% vote in the first round on 10 April was at the upper end of the first-round scores achieved by most recent French presidential victors, though a world away from De Gaulle’s own 45% in the first round in 1965. In almost any other democracy, Macron’s 17-point margin of victory over Le Pen in the second round would be regarded as spectacular. But the results obscure what is wrong with the system. The two-round electoral mandate remains De Gaulle’s ambivalent legacy to French politics. His constitution gives presidents an authority that is both real (because it produces a clear winner) and illusory (because most people vote for someone else or don’t vote at all). The combination of that mixed mandate and executive power survives – for now. Yet it is an increasingly unstable one, especially in a country with France’s vigorous tradition of street politics. The system gives a voice to the diverse political stances characteristic of any modern democracy, while also covering them over. Electoral fragmentation is now an embedded feature of many 21st-century democracies. The divide in the first round of France’s presidential elections is like many similar ones among today’s European electorates. Last year’s German election was a particularly notable case. Topically, it also bears comparison with the divide in the run-up to Northern Ireland’s crucial assembly election next week. Many people are still tempted to treat politics as a battle between a large party of the left and a large party of the right. Many facts and figures belie this. In France, the four strongest finishers on 10 April captured 28%, 23%, 22% and 7% of the vote respectively. In Germany last year, the top four had 26%, 24%, 15% and 13%. In Northern Ireland (where the divides are not merely left-right), the latest polls show the four strongest parties are Sinn Féin on 26%, Democratic Unionists 19%, Alliance 16% and Ulster Unionists 13%. If Northern Ireland was electing a president under the French two-round system, this would mean there would be a second-round run-off between Michelle O’Neill of Sinn Féin and Jeffrey Donaldson of the DUP. That’s never going to happen, fortunately for Northern Ireland, because it would fuel a wholly sectarian dynamic. But it illustrates the inbuilt weakness of any electoral system, such as France’s, which translates minority support in the first round into a majoritarian mandate in the second. Northern Ireland’s very deliberately does not do this. It is rooted instead in the power-sharing principles and practicalities of the 1998 Good Friday agreement. This means that Sinn Féin’s expected first-place finish on 5 May would in reality be a victory of only a limited kind, albeit one with headline-news value worldwide and enormous symbolic meaning. That a territory that was created 101 years ago specifically to cut itself adrift from Irish republicanism should now elect a first minister from that same republican tradition is a milestone in anyone’s language. In those circumstances, though, the chances of the DUP agreeing to nominate a deputy first minister to work alongside O’Neill as first minister are remote. The same goes for the possibility of an early referendum on Irish unification. It is much more likely that the failure of the two main parties to work together (both of them may lose share of the vote since the last contest in 2017) will hasten the return of direct rule from London. If the UK government also shreds the Northern Ireland protocol, as Lord Frost urged it to do in a speech on Wednesday, power-sharing will again be left high and dry. Electoral systems matter. Germany, France and Northern Ireland illustrate some of the different ways this works. But they are not unusual. Electoral systems do not only reflect politics; they also shape the way politics is conducted. This goes for Britain too. The first-past-the-post system routinely turns an electoral minority into a constituency victory or a parliamentary majority. It is part of the reason why our politics looks and sounds the way it does. And it is increasingly indefensible in the pluralistic political climate of the 21st century. Britain’s next general election may return another hung parliament, in which the electorate votes for change but does not give any one party the mandate to govern. Replacement of the first-past-the-post system will never be a big doorstep issue. But it would be a transformative act for Britain, and one that a minority Labour government could deliver. This country’s damaged democracy is in need of its own form of power-sharing. It would be a huge step towards creating anew a certain idea of Britain. Martin Kettle is a Guardian columnist