( dpa ) - Buoyed by strong showings in state elections, the Left Party is generating turbulence in German politics out of all proportion to its size.
As long as the party's electoral successes were confined to the formerly communist east - where it is the largest party with almost 30 per cent of the electorate - the major parties could use it as a whipping post.
But the Left's emergence as a political force to be reckoned with in important western states - where it now has backing of around 8 per cent - has created a headache for the Social Democrats (SPD) in particular.
"The success of the Left have recognizably altered the discussions on policy in the other parties," says Renate Koecher of the Allensbach institute for public opinion research.
"The efforts of the SPD to re-establish itself with leftist positions have brought it little benefit but have rather strengthened the Left," the professor believes.
In state elections at the end of January in Hesse and Lower Saxony, the Left entered the state legislatures for the first time, gaining 7 per cent in Lower Saxony and just clearing the 5-per-cent electoral hurdle in Hesse.
In Sunday's election in Hamburg it is predicted to secure around 8 per cent, entering the city-state's legislature, again for the first time.
For a party with its origins indelibly tainted by the East German communist dictatorship that collapsed less than 20 years ago, these are significant successes - successes that spell trouble for the SPD.
The SPD is prepared to cooperate with the Left in the eastern states and does so in the city-state Berlin, where the SPD and Left form the ruling coalition.
But it has thus far refused cooperation in the west, aware that it could endanger its claim to occupy the centre ground of German politics.
Another reason given is that the Left's political organization in the west is unstable and its candidates are unreliable political partners.
The SPD's misgivings in this regard were confirmed as recently as last week, when Christel Wegner, a communist who was elected to the Lower Saxony legislature in January on the Left ticket, defended the Berlin Wall and the discredited East German security service, the Stasi, on national television.
This was too much even for the Left, who promptly threw her out.
Nevertheless, on Thursday it was widely reported that SPD federal leader Kurt Beck was ready to reverse a previous promise and give his approval to cooperation with the Left in Hesse, a key western state that contains the financial centre of Frankfurt.
Beck vehemently denied the reports. "There will be no agreements of any kind with the Left, not even toleration," he said.
And the SPD candidate in Hamburg, Michael Naumann, was similarly emphatic.
"No, no, no," he said in answer in answer to a question from Deutsche Presse-Agentur dpa on whether the SPD would consider entering into an agreement with the Left after Sunday's poll.
But the rumours would not die down.
Correspondents claimed to have heard Beck say at a dinner on Monday that he would allow Hesse's SPD leader, Andrea Ypsilanti, to seek election as premier with the aid of Left votes to break a deadlock in the state legislature created by the inconclusive January 27 election.
According to the reports, Ypsilanti would head a minority government of the SPD and Greens tolerated by the Left, before calling early elections in the hope that the SPD's position would be strengthened.
In Hamburg, where the polls suggest another inconclusive result, the Left is mounting a strong challenge. Its fortunes have been boosted by a tax evasion scandal taking in the upper echelons of German business.
If there is no clear result in the northern port on Sunday, the Social Democrats could once again find the route to power blocked by their refusal to talk to the Left.
And that could force a rethink on their policy towards their dealings with the party in western states. Beck may yet decide to backtrack on previous pledges.