For 70 years, Germany stood apart as the only European democracy without a rightwing party of weight. Nazis old and new tried, but they faded away one by one. The centre held, with the moderate-right Christian Democrats (CDU) and the reformist-left Social Democrats (SPD) placidly alternating in power or sharing it.

Germany has now gone mainstream. Like everybody else in Europe – from Portugal to Poland – Germans now live with a rightwing populist party. Alternative für Deutschland (AfD) has barged into parliament as the third-largest force – two generations after the demise of the Third Reich.

But the Nazi analogy is deceptive. Like its European counterparts, the AfD is an ideological hodge-podge, defying classic left-right categorisation. The revolt against politically correct speech and assertive minorities, shading off into racist shibboleths is rightwing. The clamour for social protection for the “little man” is left. Angst about immigration and globalisation, coupled with hostility to Brussels, is both left and right.

To put it brutally, the reformist left is losing its customer base

The common denominator is resentment and protest. So think Marine Le Pen and her Front National, Geert Wilders and his Freedom party (PVV), Ukip, Donald Trump, and of course Hungary and Poland where authoritarian populists are running the governments.

Anger beats agenda. Only 34% of people who voted for AfD did so with conviction. Twice as many voted that way because of their “disappointment” with the established parties. Elsewhere in Europe, this pattern applies. You can label it: “We feel betrayed and abandoned.”

But these sentiments highlight a much bigger picture: the precipitous decline of those large centre-left and centre-right parties that have governed Germany and Europe since the second world war. Angela Merkel’s CDU/CSU took the hardest hit. Compared with their last take in 2013, the German conservatives lost 8.5 points. The SPD shed only five, which is no consolation, though. The smaller loss merely masks the tragedy of the party of Willy Brandt and Helmut Schmidt that once won 46% of the vote. It is now down to 20.5%.

The SPD is battling long-term decay. Here, too, Germany is not alone; the signs of decline for social democracy stretch across Europe.

In Italy, the once mighty Italian Socialist party (PSI) is no more. The Socialist party in France used to be strong enough to heft François Mitterrand and François Hollande into the Elysée Palace. In this year’s presidential race, they captured only 6.4% in the first round. In Scandinavia, the moderate left has taken a beating. Norway is ruled by conservatives and populists. Sweden by the centre-right.

The Dutch Labour party (PvdA) has plunged from 19% to less than 6% in five years. A similar fate has befallen Greece’s Pasok. To take in the whole panorama, imagine a map of Europe. Twenty years ago, the map was mainly covered in red, the traditional colour of social democracy. Today, only five countries are inked in red.

Then look farther afield. At first sight, Britain sticks out as the great exception because Jeremy Corbyn’s Labour almost edged out the Tories in the June election. Arguably, the verdict was more anti-May than pro-Labour.

In the US, the Democrats seem to fare much better, given their majority of the popular vote in 2016. Now look again, at middle America. Since Barack Obama’s first victory, the Republicans have gained 1,000 additional seats in the state legislatures, and 34 out of 50 governors are Republicans. Trump, who has moved into the White House, would feel right at home with Le Pen, Wilders and the German AfD.

Where once a proud Socialist International bestrode the global stage now it is the Populist International stretching from the Pacific to the Baltic Sea. What happened?

Historically, social democrats rose to power in tandem with a rising working class. Now, this once mighty force is shrinking along with manufacturing as a share of GDP. In the past 50 years, that portion has roughly dropped from 35% to 15% throughout the west. To put it brutally, the reformist left is losing its customer base, and it shows in all recent elections.

It is also losing its unique selling point, which is redistribution and the all-providing state. Take Martin Schulz, the SPD’s hapless candidate for chancellor. His message was “social justice” – taking from the rich to help the poor through taxes and benefits. But today’s German workers are middle class, and the highest tax bracket bites at €50,000 – the salary of an upper-level teacher or skilled worker.

These folks do not look forward to more taxation – not in a country where the government takes in almost half of GDP. Income inequality, as measured by the Gini index, is lower than in France, Italy or Britain. So the SPD must find another selling point. Yet the SPD is trapped by its traditions. It cannot switch to Trumpist nationalism, protectionism and border closures, not when Germany lives off exports in a way no other western nation does. Nor can it drop gender and minority advocacy, which is part of its modern DNA. The same holds true for the SPD’s western counterparts.

That said, do savour the upside. The AfD took 13% of the vote in the German elections. This means that 87% supported the established democratic parties. This is a lot more reassuring than the election outcomes in the US, Holland or France – not to speak of Poland and Hungary.

• Josef Joffe publisher-editor of Die Zeit