A Commercial for the opposition Social Democrats (SPD) now showing at cinemas throughout Germany features a four-man team aboard the Starship Enterprise about to be beamed up to an endangered planet. The first three vanish successfully but the last member of the team, Chancellor Helmut Kohl, is left behind as a voice-over announces: "Insufficient energy".

The message to voters is, clearly enough, that the beam-resistant, 68year-old Chancellor is not equipped for the modern age. But some Social Democrats fear that another interpretation may be more appropriate: no matter how hard they try, they cannot budge Dr Kohl from the position he has held for 16 years.

Most commentators still expect the popular Social Democratic candidate, Gerhard Schroder, to become chancellor after Germany's federal election on September 27th. But two opinion polls last week showed Dr Kohl's Christian Democrats (CDU) closing the gap on the SPD to just three percentage points in what could be the start of a dramatic comeback for one of Europe's most formidable political campaigners.

"Others win the opinion polls, I win elections," the Chancellor likes to boast.

But the task facing Dr Kohl in this election is more daunting than any before, with 73 per cent of Germans telling pollsters that he has already been in power too long. Dr Kohl is the only one of the 12 European leaders who signed the Maastricht Treaty still in office and the next longest-serving leader. Belgium's Jean-Luc Dehaene has been in power for just six years.

But the Chancellor's unrivalled profile on the international stage has disguised a steady loss of influence in domestic politics, manifested in his party's defeat in successive state elections and his failure to push through a crucial reform of the tax system.

Mr Schroder is the most impressive candidate the Social Democrats have had since Helmut Schmidt, the man Dr Kohl ousted from office in 1982. And the SPD campaign has been sophisticated and disciplined while the Chancellor's allies have often appeared to be in disarray.

There are signs, however, that Germans are tiring of the upbeat message presented by Mr Schroder and that they may be confused by the techniques he has adopted from the successful campaigns of President Bill Clinton and the British Prime Minister, Tony Blair.

One tactic that appears to have backfired is Mr Schroder's decision to nominate Jost Stollmann, a self-made computer entrepreneur, as his shadow economics minister. Mr Stollmann, who was a member of the CDU until 1986, says he has no intention of joining the SPD. He delights in upsetting traditional Social Democrats by calling for the tax system to be dismantled and for shops to be allowed to open around the clock.

Mr Schroder hopes that such business-friendly sentiments will attract the floating middle-class voters he has identified as essential for winning the election. But he is careful to avoid presenting Mr Stollmann's ideas as party policy.

"The more trade unionists say that they are unwilling to put Stollmann's plans into effect, the more support the SPD gains among voters in the centre," according to Klaus Zwickel, head of IG Metall, Germany's biggest union.

Mr Schroder's advisers use focus groups to test every campaign initiative, and the SPD's candidate has more friends in the media than Dr Kohl, who treats most journalists with unconcealed hostility and contempt. But Jurgen W. Falter, professor of politics at the University of Mainz, believes the Social Democrats' strategy may yet come unstuck.

"The SPD is creating a credibility gap with its double strategy. When voters feel that someone is having them on, they react with great sensitivity," he said.

Dr Kohl will spend much of the remaining seven weeks of the campaign contrasting the SPD's new, sober image with the wilder policies espoused by the Greens, Mr Schroder's favoured coalition partners. Support for the environmentalist party has plummeted from 12 per cent earlier this year to 6 per cent, just above the minimum required to win seats in the Bundestag.

Most commentators blame Green pledges to triple the price of petrol, impose a speed limit on the autobahn and close all nuclear power stations within four years. The Greens are confident that enough voters will respond to their straight talking to return the party to the Bundestag but hopes of sharing power are starting to fade.

Perhaps the biggest advantage Dr Kohl enjoys is the innate conservatism of Germany's voters, who have not voted a sitting chancellor out of office since the end of the second World War. Konrad Adenauer stayed in power for 14 years with an election slogan promising keine Experimente (no experiments), and there is little evidence that today's voters are more adventurous.

The economy is beginning to boom, and Dr Kohl is confident that unemployment will fall below the psychologically important figure of four million in time for the election. Although few Germans credit the Chancellor with the economic recovery, their new mood of optimism may make the need for change at the top appear less urgent.

Besides, many voters may feel that their world is changing dramatically enough without adding a change of government. All parties acknowledge the necessity of radically reforming Germany's tax and pension systems and of making labour more flexible.

The introduction of the euro next year will create a new economic governance for the whole of Europe, with uncertain consequences for Germany. German firms are better placed than most to take advantage of the euro, but savers fear that their investments could lose value if the new currency is not as stable as the deutschmark.

Dr Kohl has frequently linked his political fate with the successful introduction of the euro and, at next month's election, Germans may decide to bid farewell to the Chancellor and the deutschmark at the same time.

German voters regard both Dr Kohl and the deutschmark as part of their post-war heritage. Like the German currency, the Chancellor embodies the virtues of resilience, reliability and security in an unstable world.

Most Germans agree that both Dr Kohl and the deutschmark have reached their sell-by date, but the Chancellor is calculating that, in the solitude of the polling booth next month, enough will decide that losing two national institutions in one year would be just too much to bear.