The Social Democrats' conference in Dortmund on Sunday, to approve the party platform for the national election on September 24, took place in the shadow of yet another drop in the polls.

After Martin Schulz inherited the party chairmanship in late January, the Social Democrats attracted 32 percent in opinion polls, briefly topping Angela Merkel's conservative CDU-CSU. Commentators spoke of a "Schulz effect." But since then any such effect has been negative. Opinion polls released this weekend put the SPD at 23 to 24 percent - a whopping 15 to 16 percent behind the conservatives. In addition, the SPD has lost three regional elections to the CDU this year.

So priority number one for the Social Democrats is to stem the bleeding. In the conference-opening speech, the SPD's Manuela Schwesig repeatedly hit on the theme of improving social benefits - the party's lead slogan is "It's time for more social equity." But first she stressed that the party, which unanimously elected Schulz its candidate for chancellor in March, was still behind its leader.

It was a tacit acknowledgement of growing doubts that Schulz may not be up to the task of toppling the current chancellor. It's difficult to imagine the CDU feeling the need to re-avow its support for Merkel, who is seeking her fourth term in office.

The election is only 90 days away, which means that Schulz and the SPD need to start mounting an unlikely comeback. At the party conference they turned to the last Social Democrat to win a national election and a man who, 12 years ago, almost denied Merkel Germany's highest office.

The Schröder effect?

When he took over the party, Schulz hinted that he would seek to roll back some of the Agenda 2010 cost-cutting reforms to Germany's social welfare system enacted under the former SPD Chancellor Gerhard Schröder (1998-2005).  Schröder, a supporter of Schulz's predecessor Sigmar Gabriel, pointedly stayed away from Schulz's coronation in Berlin in March.

Schulz has long since stopped criticizing Agenda 2010 and Sunday's party conference provided the chance for reconciliation: Schröder appeared in Dortmund to rally the troops and suggest that a comeback is indeed possible.

The 72-year-old began his speech by citing headlines that predicted an historic defeat for the SPD in 2005.

"When I read the newspapers today, it all sounded very familiar," Schröder said, pointing out that in 2005, the party had erased a double-digit deficit in opinion polls to lose to the CDU by a single percentage point. "What was possible then is also possible now."

Schröder's speech brimmed with the sort of full-throated, salt-of-the-earth exhortations that Germans came to know and sometimes love when he was chancellor. If the SPD is to avoid a fourth-straight national electoral defeat, Schulz is going to have to show that he can muster the same sort of passion. Ahead of the conference, Schulz's speech was already being billed, in the words of Spiegel magazine, as a "last chance."

Schulz in the spotlight

The 61-year-old SPD candidate, a former EU parliament president who spent 13 years working in Brussels and only rejoined German national politics last year, began by attacking the conservatives and Merkel.

He cited the idea of "asymmetrical demobilization" and told an anecdote of a public opinion specialist advising Merkel to avoid taking any concrete positions in previous campaigns. He also accused the CDU-CSU of purposely trying to lower voter participation

"It was a successful model in 2009 and 2013, but no longer in 2017," Schulz said. "I call this an attack on democracy."

He went on to say that the SPD party platform is full of investments for the future, particularly in the areas of digitalization and education. It took him roughly twenty minutes to find his way to the topic of social equity.

"People can trust us because we're the party that respects what they've achieved in their lives and puts it front and center," Schulz said.

A jibe at Merkel and Trump

Supporters will say that Schulz's 80-minute speech was wide-ranging. He touched on everything from Turkey to right-wing populism, to women's rights, to free kindergarten and universities for all. Critics might object that it lacked focus.

Schulz drew some of his most enthusiastic applause when he criticized Merkel's statement, largely interpreted to express the growing distance from the Donald Trump-led US, that the times when Germany could "rely on others" were "to a degree over."

Schulz called that statement non-specific and hedged, contrasting it with Schröder's clear refusal to support the US-led Iraq War in 2003. He also cited statements from that period by Merkel, then the leader of the opposition, criticizing Schröder's position. The implication was that the SPD would stand up to Trump in a way Merkel would not.

Schulz was much more direct in his attacks on Merkel than in previous speeches, and the SPD delegates gave his words the expected standing ovation. What remains to be seen is whether they can dent - or even draw a reaction from - Merkel.

The SPD's double bind

The SPD is facing twin challenges: to depict themselves as an alternative to a party with which they've governed for the past four years and to convince voters that what are usually regarded as Merkel's strengths are actually weaknesses.

The party platform that delegates approved on Sunday, with only one abstention, proposes a moderate redistribution of the tax burden from lower and middle income wage-earners to the wealthy, as well as modest increases in state benefits for families, the unemployed and pensioners. But it's hardly a declaration of war on the policies of the grand coalition that the SPD has formed as junior partner with Angela Merkel and the CDU. As someone who was not a member of it, Schulz has more freedom to attack the current government than Gabriel would have had, but the SPD lacks a central, hot-button issue to distinguish itself from the conservatives.

Without such an issue, it will be very difficult to penetrate Merkel's aura of invincibility and inevitability and portray her, as one speaker at the conference put it, as a "cold politician of power." The SPD wants to attack the chancellor as a woman with no agenda or ambition beyond continuing to govern, but Merkel's pragmatic centrism and her willingness to wait and see before committing herself are among her most popular qualities.

"Those who have no platform have no plan for the future," Schulz said at the end of the conference. "We do have a plan for the future."

There may be something to the Social Democrats' criticism of Merkel, but German voters seem to like the chancellor the way she is. Moreover, her most controversial policy, the one on which she is most vulnerable, is her welcoming stance toward refugees, and that is not a position the SPD can or would want to attack.

Merkel's personal popularity as a level-headed centrist leader has put her in the enviable position of not having to campaign in any serious way thus far. Until the SPD figures out a way to damage Merkel, and the polls reflect that, the chancellor has no incentive to debate Schulz or treat him as an actual rival. And without opportunities to be seen on the same level as Merkel, it will be difficult for Schulz to gain the stature he needs, in the eyes of the electorate, to be elected in September.