Social Democrat leader Sigmar Gabriel's decision not to compete in September's elections puts Martin Schulz in the hot seat, with the daunting job of trying to stop Angela Merkel from a fourth term as German chancellor.

Schulz, whose term as president of the European Parliament formally ended just last week, has a slender chance of winning. The SPD are way behind in the polls and appear destined to either prop up another conservative-led 'grand coalition' or languish in opposition.

Here are five ways, based in part on numerous interviews with Schulz and SPD officials, that the German Left's new standard-bearer is going to set up about to pull off the upset.

1) Get the SPD into a fighting mood

Germany's oldest political party has had a frustrating four years. The Social Democrats managed to push through everything they were promised by the CDU in the coalition negotiations back in 2013, and Gabriel has outfoxed the chancellor over the German presidency, with the SPD's Foreign Minister Frank-Walter Steinmeier likely to get the job next month. But politics are about perceptions as much as results and nobody seems to have noticed: According to the latest polls, only a fifth of the German electorate is likely to choose the SPD on September 24.

"SPD is a courageous party and we want our confidence from our discussions to carry over to our society" — Martin Schulz

"If I ran now, I would fail, and with me the SPD," Gabriel was quoted as saying by Stern magazine. Internal SPD polls made it clear that Schulz scores much better among the party faithful, as a moderate who nonetheless gets the support of the SPD's left wing, who are eager to ditch Gabriel and break free of Merkel's embrace.

"We've got a better a chance with Martin Schulz in these times," Karl Lauterbach, the left-wing SPD lawmaker, told broadcaster WDR, adding that he was disappointed by the 'grand coalition' and wanted a fresh start.

One poll last year, for ARD, suggested that Schulz is as popular as Merkel with an approval rating of 57 percent. As the European Socialists' Spitzenkandidat for the presidency of the European Commission in 2014, he came second to Jean-Claude Juncker overall but scored 28 percent among Germans who voted — a result that would put the SPD in seventh heaven if they replicated that in September's national elections.

"SPD is a courageous party and we want our confidence from our discussions to carry over to our society," Schulz said in a press conference with Gabriel on Tuesday in Berlin.

2) Unite the left

Even if the conservatives look impossible to beat head-on, the SPD could try to outflank it on the left by building a "Red-Red-Green" governing coalition with the Greens and the Left party (or even the liberal Free Democrats.) That means Schulz should keep channels of communication open with moderate leftists like Thuringia's state premier Bodo Ramelow and Dietmar Bartsch, co-leader of the Left party in the Bundestag group.

The maths only work for the Red-Red-Green if the far-right Alternative für Deutschland doesn't eat too big a slice of the electoral cake.

He shouldn't waste his time trying to court hardcore leftists like Bartsch's co-leader Sarah Wagenknecht or her husband Oskar Lafontaine. If the left wants a chance of tasting power, it will come to him — if he uses the campaign to highlight what differentiates the SPD from the CDU, what makes Schulz different from Merkel.

However, the maths only work for the Red-Red-Green if the far-right Alternative für Deutschland doesn't eat too big a slice of the electoral cake. Currently, polls put it third place ahead of the Greens and Left. That means Schulz must do all he can to shift the focus away from AfD leader Frauke Petry, and some proper fights with Merkel might indeed help to distract attention.

3) Avoid migration

The SPD electorate are notoriously hard to define, divided — like other parties with working-class roots across Europe — between those who want the state to be tough on migrants and others who would "crucify us" (in the words of one SPD leader) if the party leadership tried to diverge from Merkel's controversial Willkommenskultur.

When Schulz outlined his political priorities in an interview last year, he said he wanted a legal framework in the EU that ensures asylum is not granted to people who are not victims of political persecution, while balancing the interests of people who come to Europe to seek a better life for themselves and their children.

That's easier said than done, of course. But as Schulz and Merkel prepare to compete with each other on podiums up and down the country in coming months, they share at least one common goal: Avoid the migration issue where possible, and if it can't be avoided, point to European solutions that are in the works and hope, above all, that the numbers of new arrivals continue to drop.

On Tuesday, Schulz said: "There will be no Europe bashing from me, no hatred for minorities," he said. "I will fight populists and anti-democrats. Social democrats have always fought against enemies of democracy."

4) Talk to ordinary Germans

If the AfD does manage to get into the Bundestag for the first time in September, it represents a threat above all to anyone trying to form a majority at the left of CDU of the way it would divide up the electorate. That means Schulz must address himself to the potentially disaffected sectors of the working and middle classes at risk of being lured away by the AfD's populist message.

In conversations over that past few months, Schulz has given glimpses of an electoral strategy that would focus on giving ordinary Germans the feeling that their work, their tax money and their respect for the rules is worthwhile.

"We want hard working people who stick to the rules to be able to live well in Germany" — Martin Schulz

One of his favorite lines, repeated throughout the year in various formulations, has to do with the German obsession with fair taxation. "When people see that their taxes are used to give their children a future, they buy into it. When they see that their taxes are used to pay opportunists, they turn away."

Another version, adapted to wherever Schulz is delivering his speech, is that it's unacceptable for the local pub in [fill in the name of the relevant German city] to pay taxes when Starbucks doesn't.

"We want hard working people who stick to the rules to be able to live well in Germany," he said at Tuesday's press conference. "We want people to feel respected, to have a good future for their children … that they have the same, fair opportunities to achieve in our society. SPD has a claim to leadership on these issues in this country."

5) Play to your strengths

Schulz's detractors often say he has no experience in German politics other than his years as mayor of Würselen, his small hometown close to Aachen, and tend to speak disparagingly of his 23 years in the European Parliament. In one opinion poll late last year, one in four Germans said they had never heard of him — though, on the flipside, that implies that three-quarters of them have.

In a country where politicians like to make a fuss of their doctorates, Schulz is a high-school drop-out who didn't go to university. He has fought off alcoholism — successfully — and is no stranger to personal criticism from sectors of the media. However, he also knows how to leverage his position as Germany's most influential man in Brussels, appearing in lots of TV talk shows and interviews and making the most of the photo opportunities. Fluent in English and French and few more European languages, he can more than hold his own at top international gatherings like Davos, where last week he met U.K. Chancellor Philip Hammond and U.N. Secretary General Antonio Guterres, and picked a fight with Dutch Prime Minister Mark Rutte.

And if the worst comes to the worst, and the SPD is condemned to another four years playing junior partner to Merkel's conservatives, Schulz's Brussels years show he can flourish in a 'grand coalition.'