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German Chancellor Angela Merkel's election rival took more fire Friday for a magazine cover showing him making the vulgar middle-finger gesture, with many questioning his fitness to lead the nation.

While some defended the photo as a human and ironic gesture, most commentators questioned the political judgement of Peer Steinbrueck, 66, in allowing himself to be shown flipping the bird and then authorising the picture's publication.

The striking black-and-white portrait of the Social Democrat was published in a newspaper's weekly magazine but had already gone viral online, sparking a storm of social media ridicule ahead of the September 22 vote.

The most damning commentary, predictably, came from the ruling conservatives, who said it showed Steinbrueck -- whose personal approval ratings lag behind Merkel by 20-30 points -- had already thrown in the towel.

"Someone who presents himself like that before the elections doesn't want to become chancellor," said lawmaker Wolfgang Bosbach of Merkel's Christian Democratic Union (CDU).

The photo of Steinbrueck -- a former finance minister and self-styled "straight talk" politician -- was taken more than a month ago for the regular photo feature in which subjects are asked to respond non-verbally to questions.

The gesture by a sneering Steinbrueck was his response to nicknames such as "Problem Peer" given to him by the media over blunders and gaffes that have hobbled his campaign.

Communications expert Stephan Lermer judged the picture "a pubescent gesture, extremely vulgar" and predicted it would become iconic and define Steinbrueck, the way Albert Einstein is best remembered for the photo with his tongue out.

"As Germans, we have to cringe," he said. "It will cost him votes. The gesture is unappetising. It lacks class. It will especially turn off elder, conservative voters ... He thought he could score with irony, but the Germans are a serious people."

Social Democratic Party (SPD) chief Sigmar Gabriel had jumped to the defence of the candidate, saying on Twitter that Steinbrueck "used an ironic photo interview to ironically show emotions".

Steinbrueck again defended himself Friday, light-heartedly saying that the campaign had to be kept "a bit funny", that he had just been himself and acted "spontaneously".

"When you do this interview, you don't have a lot of time to think... you have to react quickly," he said during a telephone conversation with a well-known entertainer, released online by the SPD.

In the photo essay with the Sueddeutsche Zeitung's magazine, he said, "I hope that the country will understand that the grimaces and gestures relate to the question, and show a sense of humour."

But many were not amused.

"Angela Merkel also has a sense of humour, but you wouldn't see such a photo or gesture of her, that's the difference," said CDU lawmaker Steffen Kampeter.

"Peer Steinbrueck's personality means there is a limit to his fitness for political leadership posts. He's a funny guy, but chancellors face extreme situations and have to be aware every second that they represent Germany."

Far-left Linke party chairman Bernd Riexinger considered the picture the "official end of the chancellor candidature of Peer Steinbrueck".

The Berlin daily Tagesspiegel didn't buy the irony defence, commenting that "the finger is a question mark, the candidate a riddle".

Editor Stephan-Andreas Casdorff said: "If he thinks he will speak to young voters like that -- he is 66 -- then I think that's a mistake. Because young people will say, 'that's unreal! He is 66'."

The Frankfurter Allgemeine Zeitung daily imagined how, after a Steinbrueck win, the offending image could be used in protest placards from Greece to Afghanistan.

"A picture speaks more than a thousand words," said a commentary. "The gesture of the chancellor would tell the world just one thing: 'Fuck you.'"

The image stood in stark contrast to one of Merkel, often called Europe's most powerful leader, on the cover of Friday's Economist weekly, with the headline "One woman to rule them all".

The chancellor's spokesman Steffen Seibert, who is supposed to stay outside party politics, said about the middle-finger picture: "I can only say that I have no words for this".