* Merkel and SPD rival in lone TV campaign debate on Sunday
* Steinbrueck could attack chancellor on Greece
* Comedian moderator known for outrageous questions adds new twist (Adds Steinbrueck on Syria)
By Noah Barkin
BERLIN, Aug 29 (Reuters) - Germany's Social Democrats (SPD) are citing Mitt Romney's dominating performance in his first debate against Barack Obama last year as a model for Sunday's TV clash with Angela Merkel to spice up the last few weeks of a lacklustre campaign.
Peer Steinbrueck, the SPD candidate for chancellor, is trailing badly in opinion polls, but his rhetorical gifts could give him an edge over Merkel in their only televised showdown before the Sept. 22 election, and he could be helped by the inclusion of an irreverent entertainer on the question panel.
He has promised to confront her over Greek bailouts, but Steinbrueck will also want to avoid appearing too aggressive in the 1-1/2 hour prime-time clash with Merkel, whose popularity rests in part on her modest, reserved style.
The former finance minister has a quick wit but he can seem arrogant and overbearing. Like U.S. Republican challenger Romney, polls show he suffers from a "likeability" problem.
"I don't need to rehearse," Steinbrueck said on Thursday. "I get the impression that my appearances are more interesting and lively than Frau Merkel's. With me, it rocks."
Ahead of the face-off, Steinbrueck's schoolteacher wife Gertrud has suggested he take deep breaths before launching into critiques of Merkel - advice he has promised to follow.
"An experienced chancellor is going up against a candidate with better rhetorical skills and nothing to lose," said Maybrit Illner, a political talk-show host for public broadcaster ZDF who is one of four moderators for the debate.
"His goal must be to dent her credibility, to insult her in a way that doesn't make him look bad. This is not an easy task."
Adding a twist to the debate is Stefan Raab, one of Germany's most popular entertainers, comedians and musicians, who will join Illner and two other more traditional TV anchors in lobbing questions at the candidates.
The trained butcher and law school dropout is a phenomenon on TV thanks to his cheeky style and outrageous questions, like when he asked a top swimmer if she ever urinated in the pool.
Raab, who came fifth in the 2000 Eurovision song contest with his nonsensical "Wadde hadde dudde da?", could make life uncomfortable for both candidates, but especially Merkel, who is not known for her spontaneity and only reluctantly accepted him as a moderator.
Raab's presence could attract younger viewers, swelling the audience beyond the 14 million who watched the last chancellor debate between Merkel and Frank-Walter Steinmeier in 2009.
The duel four years ago was widely panned in the media as a bore, with top-selling Bild newspaper dismissing it with the headline "Yes we gaehn", a play on the Obama campaign slogan, replacing "can" with the German word for "yawn".
Sunday's showdown promises to be more confrontational.
Steinbrueck, 66, is a far feistier character than Steinmeier was. He has accused Merkel of trying to hide the costs of a new Greek bailout from voters.
But he may be wary of trying to play the "peace card" on Syria, as SPD chancellor Gerhard Schroeder did so successfully during his 2002 campaign by staunchly opposing the U.S.-led invasion of Iraq.
On Wednesday, the chancellor stepped up her rhetoric, vowing that the regime of President Bashar al-Assad could not go unpunished for its apparent use of chemical weapons.
The German electorate has deep pacifist strains and polls show broad opposition to military strikes against Damascus. But Steinbrueck left his options open, saying on Thursday: "With me as chancellor, Germany will always be a reliable partner for our friends on foreign, security and alliance policies."
The two are likely to clash on taxes: Merkel depicts SPD plans to hike levies for top earners as a danger to the economy.
Opinion polls give her conservative bloc, which includes her Christian Democratic Union (CDU) and the Bavarian Christian Social Union (CSU), a 15-19 point lead over Steinbrueck's SPD.
That virtually guarantees Merkel will remain chancellor. But it is unclear whether she will get enough votes to continue her coalition with the business-friendly Free Democrats (FDP).
Should she fall short, Merkel would probably be forced into difficult talks with the SPD, with whom she ruled before in a "grand coalition" between 2005 and 2009.
So despite a formidable lead, Merkel cannot relax: the debate may be decisive for the shape of the next government.
Although reluctant to liken Steinbrueck to the conservative Romney, the SPD draws hope from the U.S. candidate's strong showing against Obama in the first of their three debates.
An aggressive Romney received a big boost in opinion polls after that clash, in which Obama, normally known for his charisma, seemed listless and distracted.
"I really don't want to make the comparison to Romney," SPD campaign manager Andrea Nahles told a Frankfurt newspaper. "But we are hoping for a similar impact."
What Nahles failed to mention is that Romney, despite his strong debate showing, went on to lose the election. (Reporting by Noah Barkin; Editing by Stephen Brown and Will Waterman)