The meteoric rise — and precipitous fall — of Mr Martin Schulz’s opinion poll ratings is one of the biggest mysteries of recent German politics. When he was named as the Social Democrats’ candidate for chancellor in January, a wave of euphoria swept the party.

The former bookseller who rose to become president of the European Parliament came across as an engaging man of the people. Members felt they finally had a leader who could unseat Angela Merkel and usher in Germany’s first Social Democratic Party (SPD)-led government in 12 years.

Before his appointment, the party had languished at just over 20 per cent in the polls, way behind Mrs Merkel’s Christian Democrat/Christian Social bloc. Suddenly they shot up above 30 per cent. Some polls even had them overtaking the conservatives.

But by the summer the bubble had burst, and with only two weeks to go until the election, the SPD is back where it started. Experts insist it is not his fault. The SPD’s problem lies much deeper — an opponent in Ms Merkel who has moved her conservative bloc so far into Social Democrat territory that it is often hard to tell the two apart.

“What’s the point of voting Social Democrat when you have Merkel’s CDU?” asks Mr Kai Arzheimer, a political scientist at Mainz University. “All the left’s demands — the minimum wage, gay marriage, the phase-out of nuclear power — have been implemented by a CDU-led government.”

But the Schulz slump is also part of a broader tale of a social democratic movement in crisis. The unionised working class, long the bulwark of the political left, has shrunk, and left-of-centre parties have been losing elections across Europe.

Mr Uwe Jun, a political scientist at Trier University, says social democratic success rested on a promise of upward social mobility.

“Yet in an era when social structures are becoming more rigid, it is becoming much harder to fulfil,” he says.

The left’s commitment to safeguarding and expanding social benefits also looks shakier at a time when globalisation has vastly increased competition and forced governments to cut welfare budgets. In Germany, many working-class voters have become alienated from the libertarian SPD leadership, particularly over issues such as gay marriage and refugees. FINANCIAL TIMES