In proposing to form a minority government (in coalition with the Greens), Ms Ypsilanti was breaking an SPD campaign promise that it would not co-operate with the Left Party, the successor to East Germany's communists.

ECONOMIST: Germany's Social Democrats

In return for allowing Ms Merkel to become chancellor, the SPD is to get control over more ministries than the CDU, including weighty ones such as foreign affairs, finance and the ministry in charge of labour-market reform.

ECONOMIST: Merkel clinches it, but the price is high | The

Ms Merkel remains a strong favourite to defeat the SPD candidate (and foreign minister), Frank-Walter Steinmeier, in September.

ECONOMIST: A German government shuffle

SPD, which has seen its popularity slide even as Ms Merkel harvests the credit for all the government's successes.

ECONOMIST: Germany's president

Mr Steinmeier, who is under pressure from the SPD to demonstrate some economic leadership before the election, has now seized on Ms Merkel's obstinacy as an opportunity.

ECONOMIST: Angela Merkel

But if she loses to Ms Maag she will still return to the Bundestag thanks to her number seven ranking on the SPD list.

ECONOMIST: Germany's federal election