By Tom Käckenhoff

MUENSTER, Germany (Reuters) - Peer Steinbrueck, the man chosen to lead Germany's opposition Social Democrats into an election next year as its candidate for chancellor, called for party unity on Saturday, saying it would otherwise be difficult to unseat Chancellor Angela Merkel.

Steinbrueck, a feisty former finance minister distrusted by Social Democrat (SPD) leftwingers for his centrist economic views, also repeated his opposition to serving under Merkel, a conservative, in a future coalition government.

"This federal election can only be won with the mobilization of our supporters ... Unity is important," Steinbrueck told a party rally in Muenster in the western region of North-Rhine Westphalia, where he once served as state premier.

"An SPD candidate for chancellor must reach 62 million voters," he added, signaling a pitch for the middle ground where he hopes to lure supporters of Merkel's conservatives.

In power since 2005 and buoyed by her calm and resolute handling of the three-year-old euro debt crisis, Merkel enjoys higher approval ratings than Steinbrueck, while opinion polls put her party comfortably ahead of the SPD.

A snap poll conducted by ARD-DeutschlandTrends on Friday showed Merkel had 50 percent personal backing against 36 percent for Steinbrueck. Poll participants considered Merkel more likeable and credible than Steinbrueck and also more likely to overcome the euro crisis.

But 58 percent of respondents also thought that Steinbrueck, who was named as the SPD's "chancellor candidate" on Friday, would be a good candidate for his party and he was considered stronger on the theme of social justice than Merkel.

Despite their lead in the polls, Merkel's conservatives may struggle to form a government if their current coalition ally, the liberal Free Democrats, do badly in the 2013 election. She may then have to consider a coalition with the SPD like the one she led in 2005-09 when Steinbrueck ran the finance ministry.

UNCOMPROMISING

On Saturday, Steinbrueck repeated his preference for a coalition with the Greens and his unwillingness to cut a deal with Merkel.

"I will not be won over to (serve in) a Merkel cabinet ... We want to throw out all three from this government," he said, referring to Merkel's Christian Democrats, their Bavaria-based sister party the Christian Social Union and the FDP.

To loud applause, Steinbrueck reaffirmed his plans for tighter regulation of the financial markets and also called for a legal minimum wage and increased spending on education.

Known for his acerbic wit, Steinbrueck emerged as the SPD's candidate after two other contenders, party chairman Sigmar Gabriel and former foreign minister Frank-Walter Steinmeier, stepped aside.

Saturday's newspapers said the choice of the combative Steinbrueck would make for a much more interesting election.

"On the one side stands the controlled and composed chancellor who seems to get on top of every crisis and is trusted by the Germans," said the best-selling Bild tabloid.

"And on the other is the passionate, rough-edged but also often thin-skinned Steinbrueck ... He will give everything for victory, without any Plan B, without a safety net for himself. If he loses that could be politically the end for him. That deserves respect. This time Germans will have a real choice."

(Writing by Gareth Jones; Editing by Andrew Osborn)