Martin Schulz, leader of Germany’s Social Democrats, had some strong words for a Hamburg landlord planning a huge rent rise. It was “daylight robbery”, “immoral”, the “unscrupulous exploitation of poor people”.

The outburst came during a live Q&A session on German TV less than two weeks before national elections. Renate Braun, a pensioner, had just revealed her rent was about to go up from €230 to €850 a month after a refurbishment.

But there was embarrassment in store for Mr Schulz. A presenter revealed that Ms Braun’s landlord was a construction company owned by Hamburg City Hall — which is Social Democrat-controlled. All 150 studio guests erupted in laughter.

It summed up the lingering doubts about the SPD leader. Once hailed as the party’s great hope, he has failed to revive its flagging fortunes. A recent poll by ARD Deutschlandtrend pegged SPD support at 20 per cent, its lowest score since January, and 17 percentage points behind Angela Merkel’s conservative CDU/CSU bloc.

If that is replicated on September 24, it would be the Social Democrats’ worst ever performance in a Bundestag election — surpassing its previous record low of 23 per cent in 2009.

In public, party officials exude a Pollyanna-ish confidence. “The atmosphere on the street does not reflect what the polls say,” says Ralf Stegner, SPD’s deputy leader, during an interview at a campaign event in the eastern city of Leipzig. “There are loads of young people at our rallies, and real enthusiasm. We have all the right themes — pensions, precarious jobs, kindergartens.”

But already the knives are out for Mr Schulz. SPD MPs are openly criticising him for critical mistakes during his campaign. The former European Parliament president has said he will run again as party leader, whatever the election result. But the worse the SPD does, the greater the pressure will be on him to resign.

Internal discussions are also heating up over what the post-election game plan should be. The big question is whether the SPD will remain as junior partner in a Merkel-led “grand coalition”. Many in the party are against such an outcome.

“If we do worse than the 25.7 per cent we got in 2013, then we’ll just have to go into opposition,” says one MP. “Party members really don’t want another grand coalition. Everyone just feels really used by Merkel.”

The dismay is a far cry from the halcyon days in January when Mr Schulz was named as the SPD’s candidate for chancellor and the party’s ratings shot up. For the first time in years it looked like it had a realistic shot at unseating Ms Merkel.

Yet its standing is now back where it was before Mr Schulz was anointed. “It’s true, we’ve been on a real rollercoaster ride,” says Mr Stegner.

Some of the setbacks suffered by the party have been self-inflicted. Ahead of crucial elections in the state of North Rhine-Westphalia in May, the local SPD leadership told Mr Schulz not to make any big policy announcements and leave them free to campaign on local issues. Fatefully, he agreed — and the SPD lost.

“That was a huge mistake,” says Daniela Kolbe, an SPD MP. “He should have been out there making his presence felt.” By keeping such a low profile during the NRW election, Mr Schulz’s campaign lost critical momentum.

When he finally did start campaigning in earnest, his big theme was social justice — a hard sell in a country where unemployment is at a record low. “People are feeling happy with their lot and there’s little appetite for change,” says the SPD MP.

There were other problems. In July, illness forced Mr Schulz’s campaign manager and friend Markus Engels to step down. Former SPD chancellor Gerhard Schröder’s decision to join the board of Kremlin-controlled oil group Rosneft proved hugely embarrassing. And a lacklustre TV debate with Ms Merkel failed to deliver the poll boost he had been banking on.

In the TV Q&A on Tuesday, Mr Schulz tried hard to come across as an amiable man of the people. He regaled the audience with anecdotes from his life as a policeman’s son and former mayor. He got up close and personal with his questioners, squeezing in to sit next to them and touching their hands — to the obvious discomfort of some.

One CDU activist, Marian Bracht, even coined a verb to describe his behaviour. “anschulzen: to get close to someone in an awkwardly-familiar way”, he tweeted.

The tactic doesn’t appear to be working. “I’m not convinced by him,” says Susanna, a young social worker in Leipzig. “Say what you like about Donald Trump, but at least he has charisma. Schulz has none.”