As he climbed on stage to address 10,000 supporters in Braunschweig's Stadthalle, Gerhard Schroeder was clearly relieved that this was the final rally in a six-week election campaign. Opinion polls are unanimous in predicting that the Social Democrat will remain prime minister of Lower Saxony after tomorrow's state election and most forecast an increase in his party's share of the vote.

If Mr Schroeder was looking tense, it was because tomorrow's election will determine much more than the future government of a thinly-populated state in northern Germany. If he performs well, he is almost certain to be chosen on Monday as his party's candidate to challenge Chancellor Helmut Kohl in the general election on September 27th.

"I want two signals to go out from Lower Saxony to the whole republic. One is that the Social Democrats will continue their work in putting the interests of employees first in Lower Saxony. The other must be a clear message that the Kohl era is over," Mr Schroeder declared.

The audience roared their approval, but another audience a few miles away in the little town of Stadthagen had given the Chancellor himself a similarly rapturous welcome a few hours earlier.

"Don't believe the PR messages you hear from the opposition," he told them. "You can buy PR but you can't buy character."

Dr Kohl has been a permanent presence during the Lower Saxony campaign, moving through the state to maximise the vote of his Christian Democratic Union (CDU). It is no secret that the Chancellor has little affection for his party's candidate, a 38-yearold lawyer from Osnabruck called Christian Wulff. It is equally clear the CDU has no chance of gaining power in Lower Saxony. But Dr Kohl's hard campaigning could pay off if he eats into Mr Schroeder's vote because the Social Democrat has vowed he will not be his party's candidate for chancellor if his share of the vote tomorrow falls by more than 2 per cent.

The Chancellor struck a relaxed pose on the question of who he would face in September, affecting to believe that all challengers faced an equally hopeless task.

After 15 years in power, however, and with unemployment edging towards five million, Dr Kohl's position has never been so precarious. There have even been rumblings within the CDU that, if the party fares badly tomorrow, the Chancellor may be forced to step down in favour of his wheelchair-bound deputy, Wolfgang Schauble.

Such a move is unlikely, not least because Dr Kohl rules his party with a grip so tight as to make organised opposition to him impossible.

But the Social Democrats smell blood and, after four failed attempts to dislodge the man they call Der Dicke, or the Fat One, they are confident their moment has arrived. For the first time in almost a generation, the party is united and efficient, an achievement that owes much to the efforts of its chairman, Oskar Lafontaine, the prime minister of the tiny southern state of Saar.

In normal circumstances, Mr Lafontaine would be his party's natural choice to challenge Dr Kohl and many party members would be happy to endorse him. He was in Braunschweig, alongside Mr Schroeder, whipping up the faithful into a frenzy of old-fashioned outrage against the injustice of the society Dr Kohl had created.

"The tax system is there to ensure that those who earn little should pay little tax and that those who have more should pay more tax. But the reality is that those on modest incomes pay income tax every month while the rich buy houses or ships or aeroplanes, write them off and pay nothing," he roared.

For months the Social Democrats have been preoccupied with the choice between Mr Lafontaine and Mr Schroeder, with their hearts favouring the former and their heads the latter. The two principals have remained above the fray, but the rival camps have been accusing each other of engaging in unprincipled intrigue and attempting to manipulate the nomination process.

In the course of this internal campaign, Mr Lafontaine has been cast as the traditionalist, a Lionel Jospin to Mr Schroeder's Tony Blair.

In fact, Mr Lafontaine has long been a moderniser within his party and was known in the 1980s as a member of the "Tuscany faction", so called on account of its affection for Mediterranean food and culture. His return to basic Social Democratic values has been, above all, a strategy to unite the party and, as such, it has been spectacularly successful.

In the same way, Mr Schroeder's image as a German Tony Blair, and his much-trumpeted links with business, conceal the complexity of his political makeup. As one of seven children raised by a war widow who worked as a cleaner, he remains close to his working-class roots and, unlike Mr Lafontaine, prefers to drink beer - at least in public.

Social Democrats know that September's election will be won in the centre ground of German politics and that conservative voters need to be reassured that it is now safe to dump Dr Kohl.

"Anyone who is not against us is with us, that must be our policy," Mr Schroeder told his audience in Braunschweig.

Although some dismiss him as an unprincipled opportunist, most Social Democrats acknowledge that Mr Schroeder is the only man they have who can put this policy into practice. This is why, barring a disastrous upset tomorrow, Mr Schroeder will be crowned candidate on Monday and Dr Kohl will sleep a little less easily at night.