* Merkel risks being eclipsed by centre-left partner
* SPD leader eyes bid for chancellorship in 2017
* Gabriel has won backing for green energy reforms
By Madeline Chambers
BERLIN, Jan 29 Germany's new vice chancellor Sigmar Gabriel has turned his Social Democrats (SPD) into a driving force in Berlin and so far stolen the headlines from his boss Angela Merkel, still hobbling on crutches after a skiing accident.
Determined to push through centre-left policies in Europe's biggest economy and boost the SPD's chances of winning the next election in 2017, Gabriel and SPD ministers have set the agenda on issues from pensions to energy and foreign policy.
Six weeks into office, it remains to be seen whether these initiatives translate into reality. But commentators and even some of Merkel's conservatives say that under Gabriel, the SPD has hit the ground running.
"In truth, is Sigmar Gabriel governing us?" asked the top-selling Bild newspaper. Few doubt the ambitious 54-year-old teacher from northern Germany has his eye on the chancellery.
SPD Labour Minister Andrea Nahles' presentation on Wednesday of cabinet-approved plans for generous pension reforms almost eclipsed the chancellor's flat flagship speech in parliament on her vision for the next four years.
SPD Foreign Minister Frank-Walter Steinmeier has stood up to conservative Defence Minister Ursula von der Leyen and wants to wrest European policy from the chancellor's office.
Although Merkel's conservatives emerged the biggest party in September's vote, she was forced to seek a deal with the SPD for a 'grand coalition' to rule with a parliamentary majority.
Gabriel negotiated a coalition deal that contained key SPD demands, including a commitment to a minimum wage and a boost to infrastructure spending.
Many SPD members were reluctant to get into another grand coalition after the last four-year stint sharing power with Merkel produced the SPD's worst post-war election result.
"Gabriel knows he must prove the SPD can get their projects through so they aren't burned at the next election," said Gero Neugebauer, political scientist at Berlin's Free University.
"He has worked hard and attracted much more attention than Merkel. He is fostering an image of a serious man who can do things, but it is too soon to see whether he is the ingredient that makes the cake rise."
So far, there is no sign of a fillip in the polls. A Forsa survey showed Merkel's conservatives up 1 point at 42 percent with the SPD flat at 23 percent on Wednesday.
Even though the image of Merkel on crutches may have played into Gabriel's hands, the popularity of 'Mummy', as the chancellor is widely known, is unassailable, said Forsa chief Manfred Guellner. "Even 29 percent of SPD voters trust Merkel more than Gabriel as chancellor," he said.
Probably Gabriel's biggest achievement so far has come at the economy ministry he heads. Responsible for overseeing Germany's ambitious renewable energy transition, he got cabinet backing to reform the system of costly green power subsidies, winning plaudits from industry and some conservatives.
"He has energetically embraced reform of the renewable energy law," said conservative lawmaker Joachim Pfeiffer. "And it is going in the right direction."
In a telling sign of his growing self-confidence, last week Gabriel arrived a brazen half-hour late to a cabinet meeting at a stately home just outside Berlin, keeping Merkel waiting.
He has made clear he will not kow-tow to her.
"She is not my boss... we see each other as partners," he said in an interview with Stern magazine on Wednesday.
The two do not use the familiar 'Du' form of address but rather the polite 'Sie', which he says reflects mutual respect.
They worked together in the previous grand coalition when he was environment minister and have got over a wobbly spell after a confidential text message from Merkel to Gabriel, then opposition leader, was leaked to media in 2010.
The former premier of Lower Saxony state has long been popular with the party base, more so than pragmatists Steinmeier and Peer Steinbrueck who ran against and lost to Merkel in the election. He now appears to be earning respect among SPD lawmakers, who had viewed him as unpredictable.
Never one to shy away from a fight, Gabriel took a risk by asking grassroots SPD members to back the coalition deal. It paid off, with an overwhelming 76 percent voting for it.
One conservative said he had evolved from "Siggi-Pop", a nickname earned when was responsible for popular culture within the SPD, to "Siggi-Serious".
Gabriel won admiration last year for discussing a difficult relationship with his father, a committed Nazi, and has fended off criticism for taking Wednesday afternoons off to look after his two-old daughter while his wife works at a dental practice.
"I work more than 70 hours in a week," Gabriel told Stern, adding anyone who called him a part timer "is round the bend". (Additional reporting by Gernot Heller; Editing by Stephen Brown and Tom Heneghan)