A green helicopter shot out of a bank of rain-sodden cloud and landed noisily on the shores of a lake that was once a giant open-cast coal mine. Thickset men in suits talking into walkie-talkies leapt from the aircraft and propelled a portly, owlish man with a shock of white hair and thick glasses towards a gaggle of east Germans. Frank-Walter Steinmeier, the man who aims to oust Chancellor Angela Merkel in next month's poll, strode with a resigned air towards the group hiding under umbrellas in the beer garden of a 19th-century villa in the once polluted east German town of Bitterfeld.

It was another stopover in the German Social Democratic Party's arduous campaign to make headway against seemingly irreversible odds in the run-up to this year's election and Mr Steinmeier seemed to admit that he was one of the reasons for his party's current difficulties.

His opening remarks to the small crowd conveyed the impression of someone staring into the jaws of ignominious defeat: "You know people keep asking me how I manage to cope with opinion polls like these," he said with a forced smile. "Well, people should realise that all the opinion polls are yesterday's polls and it's the future that concerns me. The election result is still wide open." The onlookers applauded weakly amid the spattering rain.

It was a brave performance for a politician who has been described as "clinically boring" and whose attempt to become chancellor has been written off by the German media as the "strangest candidacy in post-war history".

Kicking off his election campaign early this summer, Mr Steinmeier, who is Foreign Minister in the ruling coalition, proved extraordinarily capable and hopes were high of a strong performance. But with less than six weeks to go before the poll on 27 September, his party's popularity has plummeted to one of its lowest pre-election levels since the Second World War.

This week, his campaign suffered another serious setback with new and potentially damaging revelations about Ulla Schmidt, the Social Democrat Health Minister and key member of Mr Steinmeier's election team, who was only recently recruited to try to shore up flagging support. The German public had already reacted with horror to the disclosure last month that she had taken her official car on holiday to Spain, where it had been stolen. But ministry files published on Tuesday showed that Mrs Schmidt had in fact used her €90,000 chauffeur-driven armoured S-class Mercedes on numerous vacations dating back to 2004 – and not reimbursed the taxpayer for any of the 3,000-mile round trips.

An official government inquiry has been launched in an attempt to establish whether Ms Schmidt had abused her privileges as an MP, but her political opponents were already calling for her resignation. Wolfgang Bosbach, the deputy leader of Ms Merkel's parliamentary party, the Christian Democrats, said: "It was a fatal mistake by Steinmeier to take Ms Schmidt on to his election team."

Ms Schmidt protested her innocence yesterday and said she had taken the armoured car to Spain for security reasons. She accused her conservative opponents of deliberately attacking her because they feared having their plans to introduce more private healthcare exposed.

The Schmidt affair is unlikely to improve the Social Democrats' standing in the opinion polls, which show support for the party hovering around 20 per cent, while backing for Ms Merkel's conservatives remains at 38 per cent. When it comes to personal popularity, the disparity is even greater. More than 60 per cent of Germans would vote for Ms Merkel if she could be directly elected as chancellor compared to just 17 per cent for Mr Steinmeier.

Not surprisingly, the dismal state of Germany's Social Democrats is being compared to that of the British Labour Party before the arrival of Tony Blair. "Is there anyone more miserable than Frank-Walter Steinmeier?" asked the German-language edition of The Financial Times recently. Much of the blame for the party's failure is being attributed to Mr Steinmeier alone. Few would question his abilities as Foreign Minister, but as a candidate, he lacks charisma.

The weapon with which he hopes to defeat Ms Merkel is a recently unveiled plan to create four million new jobs in Germany by the end of the next decade by investing in green-energy projects and retraining the unemployed to look after the country's increasingly elderly population. Nobody doubts that the plan is a good one. The Green party has even accused Mr Steinmeier of plagiarising its own policies. But the trouble is that Mr Steinmeier has been unable to convince voters that his idea is realistic. The polls suggest that only 13 per cent of voters believe him.

Mr Steinmeier is a protégé of Germany's controversial former Social Democrat chancellor Gerhard Schröder. He spent years as a sort of top-level backroom boy, who always played second-fiddle to his boss. Mr Schröder rewarded him by making him his personal minister in the Chancellor's office.

When Ms Merkel's conservatives were forced to form a grand coalition with the Social Democrats after the election in 2005, Mr Steinmeier's abilities as a behind-the-scenes fixer made him a natural choice for Foreign Minister. But nowadays, he looks increasingly like the fall guy for a political party in irreversible decline.

The Social Democrats have not recovered from the trauma of the Schröder era. During his tenure as chancellor, thousands of loyal members quit the party in disgust over his unpopular Agenda 2010 programme of cuts, which were seen as a travesty of party principles. The party is now divided with its left wing in favour of forming an alliance with the former Communist Left party and the centre, represented by Mr Steinmeier, strongly opposed to the idea. In the meantime, the party is struggling to present coherent policies.

According to Manfred Güllner, the head of Germany's independent Forsa polling organisation, the consequences for the party are dire: "The fact is that for many Social Democrat voters, the present Social Democratic Party is no longer a party worth voting for," he says.

By contrast, Ms Merkel's popularity seems to grow by the week by a process of gradual osmosis. Initial criticism that the Protestant pastor's daughter raised in Communist East Germany was dowdy and incapable of inspiring confidence has vanished. Instead, voters see her as a steady hand at the helm in difficult times. Abroad and within the EU, she has put Germany back on the map.

The conservatives have also been given a boost through the appointment of Karl-Theodor zu Guttenberg, a 36-year-old Bavarian aristocrat as Economics Minister. Mr Guttenberg, one of Germany's youngest-ever ministers has emerged as the Merkel Boy Wonder – a figure seen as adept at handling the economic crisis and who, according to opinion polls, is even more popular that the Chancellor. With signs that the German economy is now emerging from recession, Ms Merkel can only benefit.

Back in Bitterfeld, many of those east Germans who had come to see Frank-Walter Steinmeier were there more out of curiosity than in the hope that he might win. "He's got some interesting ideas," said Gisela Müller, a local businesswoman who voted for Gerhard Schröder in 2005. "But against Merkel I'm afraid I don't think he's got a chance".

A touch of Borat to relieve the boredom

Germany's latest candidate for chancellor has ferocious buck teeth, wants to make the bunny rather than the eagle the country's national symbol and has tried to copy President Obama with his campaign slogan: "Yes Weekend".

The grotesque joke figure of Horst Schlammer, played by the comedian Hape Kerkeling, is the newest addition to the German political scene and he has succeeded in livening up one of the dullest elections on record.

His satirical film Isch kandidiere (I am a candidate) goes on general release throughout Germany today and is almost certain to be a box office hit. One in five Germans have said they would consider voting for him if his name were ever to appear on a ballot paper.

Mr Schlammer, is a Teutonic version of Sasha Baron Cohen's Borat. He sports a dirty brown raincoat, moustache and thick glasses and his clothes are normally covered with bits of food. He claims to be the deputy editor of a provincial German newspaper and recently set up his mock HSP (Horst Schlammer Party) to fight the general election. His manifesto pledges state-funded sun loungers and cosmetic surgery for all.

Mr Schlammer describes his party as "conservative, liberal, left-wing and a bit ecological". Asked about burning issues such as the financial crisis, he is disarmingly honest: "I have no solution," he admits. Swine flu? "I'm against it."

His film includes interviews with several senior politicians including Cem özdemir, the leader of the Green party, but being Germany it is less harsh on political leaders than its Anglo-Saxon equivalent might be.

The satire coincides with another joke election campaign waged by "The Party" which says it wants to rebuild the Berlin Wall and banish pensioners to the former Communist east. The Party is fighting a ruling which bans it from taking part in the election. Last week Vera Lengsfeld, a conservative MP, spiced up the election by appearing on a campaign poster in a low-cut dress alongside an almost identically dressed Angela Merkel. "We have more to offer," her slogan boasted.

"There is an election campaign and no one wants to participate," is how Der Spiegel magazine put it. "If it wasn't for Horst Schlammer and Vera Lengsfeld's breast campaign, most people wouldn't even notice."

Tony Paterson

'Super election year': Germany at the polls

* Elections to Germany's federal parliament (Bundestag), held every four years, are on 27 September, crowning what Germans call a "superwahljahr" (super election year) with more than a dozen local, state and federal polls.

* More than 60 million people will be eligible to vote.

* Half of the seats in parliament are directly elected, the rest via party lists using proportional representation. The party or coalition of parties with most seats elects the Chancellor.

* For the first time this year, the OSCE will send a team of election monitors, after a row over the exclusion of smaller parties.

* Political parties must obtain at least 5 per cent of the vote under a post-war law designed to prevent extremists from coming to power.

* Turnout in German elections is typically high. In the 2005 Bundestag elections, 77.7 per cent of the electorate exercised their right to vote.