Leading Social Democrats in Germany are engaged in a fierce battle of wills ahead of a crunch vote tomorrow over whether to endorse in-depth coalition negotiations with Chancellor Angela Merkel’s conservatives.

The party has become the focal point of a tense political drama almost four months after an inconclusive election left Germany in a state of limbo. If delegates at a special conference tomorrow vote against a grand coalition, Germany will be heading either for new elections or a minority government, neither of which is a popular choice and will leave Merkel’s political future hanging by a thread.

Martin Schulz, the SPD’s embattled leader, criss-crossed the country last week to secure party members’ backing for a deal. He said on Wednesday it was difficult to tell whether he had the support of enough delegates. Schulz said he was encouraged by signs that members had been “drawn together through our exchanges rather than torn apart” before the conference in Bonn. But the SPD faithful are riven over the issue, with some arguing that to form another grand coalition, or “GroKo”, with Merkel’s conservative alliance could further damage the party’s popularity and possibly even lead to its disintegration.

The party Schulz leads has roots that go back to the era of Otto von Bismarck, and it survived 12 tumultuous years of Nazi rule along the way. Now, it is facing an existential threat by being a junior partner in Merkel’s government once more. The rise of the right-wing populist and anti-immigrant Alternative fur Deutschland (AfD) looms large in the debate. Some in the SPD say that by siding with Merkel, the party would ease the way for the AfD — which entered the Bundestag for the first time in September — to become the largest opposition force in parliament.

Sigmar Gabriel, the party’s former leader and now the foreign minister, said the world was looking to Germany to find a swift solution. “It is no exaggeration: This coming Sunday it’s not only Europe that is looking spellbound at the SPD party conference, but many people far further afield. The world will be watching Bonn for real on Sunday,” he said.

Schulz has concentrated his campaigning efforts on Germany’s largest state, North Rhine-Westphalia, home to about a quarter of the 600 delegates due in Bonn. The state’s SPD faction — unlike others elsewhere in Germany — has declined to publicly position itself ahead of the vote.

The SPD leader, however, is no stranger to adversity.

Born in 1955 to civil servant parents, as a teen, Schulz was well on his way to becoming a professional footballer, when a meniscus tear put paid to his dreams and he fell into a state of depression, alcoholism and chain-smoking. Often referred to as a “man of the people”, he dropped out of high school and later, after his slump into depression, lost his job and narrowly escaped eviction. His brother, a doctor, is said to have rescued him from complete self-destruction. He has been teetotal since 1980.

Schulz was born in the West Rhineland village of Hehlrath and his parents’ political allegiances were divided between the SPD and the Christian Democratic Union (CDU). He has been married for 30 years and has two grown-up children.

Before entering politics, Schulz trained as a bookseller and is known to be a voracious reader, with a penchant for historical biographies; he has cited the works of Eric Hobsbawm, John Steinbeck and Giuseppe Tomasi di Lampedusa as among his favourites.

His political career began in 1987 when he became the mayor of Wurselen, near Aachen, a post he held for 11 years. He was a member of the European parliament from 1994, and head of the SPD MEPs since 2000.

In 2012 he was elected to the post of president of the European parliament, with 409 out of 612 votes. In the same year, he received the Nobel Peace Prize on behalf of the European Union. He has been a member of the SPD’s executive committee since 1999.

In 2003, Schulz clashed with Silvio Berlusconi, criticising his domestic policy. In return, the then Italian prime minister said he would recommend Schulz to an Italian film producer, looking to cast a concentration camp leader.

Seven years later, Schulz had a run-in with the British Ukip MEP Godfrey Bloom, who interrupted a speech he was giving on the European stability pact by quoting the Nazi slogan “One people, one empire, one leader” and calling Schulz an “undemocratic fascist”.

Schulz was lured back into domestic politics a year ago to lead the SPD in last September’s election against Merkel, who was seeking her fourth consecutive term as Chancellor. Uncertainty over the shape of the government in Europe’s biggest economy is worrying investors, nearly four months after that election in which the conservatives had their worst result since 1949 and the SPD their worst since the Second World War. Both parties that have dominated Germany for generations bled support to the far right AfD, which joined parliament for the first time, becoming the third biggest bloc.

Many in the SPD are wary of a re-run of the grand coalition that ruled under Merkel between 2013 and 2017. A junior role leaves them in her shadow, unable to win credit from voters. The SPD had vowed to go into opposition after its dismal election result, but was forced to reconsider in the interest of national stability after Merkel was humiliated by the collapse of three-way talks on a coalition with other parties.

Highlighting the strength of feeling, regional party branches in Berlin and the state of Saxony Anhalt have already signalled their opposition to the deal.

“A renewed grand coalition cannot be the result of talks. We reject the start of coalition negotiations with the conservatives,” said the motion by Berlin’s SPD chief Michael Mueller, which was passed. It said proposals on home-building, rents and migration and integration were unsatisfactory, and expressed disappointment over the absence of plans to shake up the health system.

However, it is difficult to predict the outcome of tomorrow’s vote as the regional results are not binding. At the Bonn congress, 600 delegates are free to vote how they wish. A rejection will likely bring the curtain down on Schulz’s leadership and pave the way for either a Merkel minority government or new federal elections.

Both scenarios bring instability — loathed by investors and Germans alike.

— Compiled from agencies