August Bebel, one of the founders of the German SPD, liked to make a robust case for preserving clear blue ideological water between political parties. “If your opponent praises you, beware!” Bebel once said. “But if he gets stuck into you, you are usually on the right way.”

Last weekend a narrow majority of members of one of the world’s oldest democratic parties appeared to judge that sound advice. In electing two candidates from its left flank to lead it, the SPD’s grassroots have called time on a period of cooperation and consensus between the party and Angela Merkel’s Christian Democrats. They may well have relished the furious verdict of conservative commentators, some of whom accused the party of lacking a sense of “national responsibility”, by jeopardising the CDU-SPD coalition government.

The surprise election of Norbert Walter-Borjans and Saskia Esken, both of whom are fierce critics of the coalition, indicated that for a narrow majority of SPD members, a tipping point has been reached. For 10 of the last 14 years, the country’s traditional centre-left powerhouse has been a junior coalition partner to the CDU, cleaving to the centre-ground and keeping Angela Merkel in office. During that time, support for the SPD has plummeted and it has found itself supplanted on the left of German politics. A poll this week saw its support plunge to 13.5%, compared with 21.5% for the Greens. Mr Walter-Borjans and Ms Esken were considered the underdogs in a runoff against the current minister of finance, Olaf Scholz, and his running mate, Klara Geywitz, who were backed by senior figures in the party. But against a backdrop of seemingly inexorable decline, members have opted for a clean break with the recent past.

This weekend the party will vote on whether to withdraw support for Mrs Merkel’s government immediately, possibly triggering a general election. With the party’s poll ratings at such a low ebb, that might be judged a step too far. But the new ideological direction of travel is bracingly clear. It resembles in many respects the journey that the Labour party has already taken in Britain. Mr Walter-Borjans and Ms Esken have vowed to lead the party and Germany out of what they have described as a “neoliberal wilderness”; their roadmap includes a new wave of investment in infrastructure, a higher minimum wage and more radical action on the climate emergency. In a significant break with German economic orthodoxy, they have also demanded an end to the so-called black zero doctrine, which commits governments to a balanced budget.

A new political era is coming in Germany. Mrs Merkel has said she will not contest the next election, after an unbroken period in office that began in 2005. The SPD’s decision to reposition itself on the left is not without risk, not least because of opposition among many of the party’s leading lights. But a moment of radical renewal was desperately required. In the 2005 federal election, which gave Mrs Merkel her first term in office, the SPD won 34.2% of the vote. The spiral of decline since then has been dizzying. Since senior figures in the CDU have sought to rule out any concessions to Mr Walter-Borjans and Ms Esken, the bid to win back the missing millions may begin sooner rather than later.