Kurt Beck, the embattled chairman of Germany's Social Democrats (SPD), tried to reassert his authority on Monday as he returned from a two-week flu-induced absence to dismiss criticism of his leadership and deny he had broken his word in allowing regional deals with the rival Left Party.

Asked if he thought he had lost credibility with his U-turn on cooperating with a protest party made up of former communists and disenchanted former SPD members, Beck told a news conference: "The only thing I lost in these two weeks of illness is five or six kilos and I'm pleased about that."

His decision to permit regional cooperation with the Left Party in western German state parliaments, and especially the way in which he communicated it in an oblique remark three weeks ago, has triggered a slump in opinion poll ratings for the SPD and him personally.

It even prompted SPD right-wingers to say he should refrain from standing as candidate for the chancellorship in the autumn 2009 election.

But the bear-like SPD chairman, looking a bit slimmer and a little worse for wear after fighting influenza and tonsillitis, dismissed internal critics. "When the cat's away the mice get a little more lively than usual," he said, speaking after a meeting of the party executive which gave him its full backing.

"I don't think I broke my word," said Beck, referring to his decision on the Left Party, a group he had steadfastly dismissed as too unreliable to cooperate with in western Germany, until his sudden change of heart three weeks ago.

"I said the aim was no cooperation with them in the west because I saw a chance to keep them out," said Beck. "But parties can and must adapt to new situations and they don't sacrifice their credibility if they do. Otherwise they would paralyze themselves."

He did, however, admit that he could have communicated his U-turn better. "It got a momentum I didn't intend," said Beck. "Mistakes are made in politics."

Slumping Fortunes

Only 15 percent of Germans now regard Beck as trustworthy, according to an Infratest poll released last week. His personal approval rating stands at just 20 percent, compared with 62 percent for Chancellor Angela Merkel, the poll showed.

"He has damaged the SPD but it won't be lasting damage," Dietmar Herz, a political scientist at Erfurt University, told SPIEGEL ONLINE.

"He's hit rock bottom and he'll start recovering now. People underestimate him, like they underestimated (former Chancellor Helmut) Kohl. He has grassroots support and the SPD doesn't have any alternatives for the leadership.

"And he could still get the SPD nomination for the chancellorship which won't be decided until next spring. A lot can happen before then."

Beck has bounced back before, advancing in opinion polls last autumn when he signalled a leftward shift  by departing from key elements of a reform agenda implemented by former SPD Chancellor Gerhard Schröder.

That strategy failed in its attempt to lure voters away from the Left Party but it has ensured Beck of support among rank and file SPD members who were never happy with Schröder's welfare cuts.

The Left Party, long well established in eastern Germany, has gained a strong foothold in western Germany in recent months, managing to vault the 5-percent hurdles to get into the state assemblies of Hesse, Lower Saxony and Hamburg in regional elections this year.

It is now represented in 10 of Germany's 16 state assemblies and Beck said the SPD was forced to review its strategy towards the Left Party as a result.

He ruled out cooperating with it at the nationwide level and pointed out that the SPD had refused to hold talks with Left Party lawmakers in the Bundestag federal parliament. "On a federal level there are unbridgeable divisions on central issues such as foreign and security policy, the EU and social policies," said Beck.

Left Party Debate Echoes Rise of the Greens in 80's

But he reaffirmed that the SPD's regional parties were free to examine whether it was possible to join forces with the Left Party on a state-by-state basis. He said the SPD would hold a party conference on May 31 to discuss relations with the Left Party.

"The SPD faces challenges resulting from the political development in Germany, we now have a five-party system in much of the country," said Beck. "This reflects realities in our society and we are facing up to this debate."

"A certain amount of criticism of how we launched into this debate is justified," said Beck. "But we would have had to tackle this issue in any case. However we had started it, it would scarcely have led to any less uproar."

Herz at Erfurt University said the SPD was now embroiled in the same kind of anguished debate it went through in the 1980s about whether to form alliances with the newly formed Greens, a colorful mixture of pacifists, ecologists and former street protesters who rapidly emerged as a fourth force in German politics.