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* Voters to go the polls in Schleswig-Holstein
* Vote is second of three state elections this year
* Social Democrats keen to regain momentum after loss
* Seeking to topple Merkel in Sept. 24 national vote
By Fanny Brodersen
KIEL, Germany, May 6 (Reuters) - Germans in the far northern state of Schleswig-Holstein vote on Sunday in the first of two regional polls that the Social Democrats must win if they are to pose a serious challenge to Chancellor Angela Merkel in September's national election.
The Social Democratic Party (SPD) enjoyed a revival in the opinion polls early this year after nominating former European Parliament president Martin Schulz in January as its candidate to run against Merkel.
But the "Schulz effect" failed to deliver in Saarland, where his party flopped in a March state poll. In Schleswig-Holstein, the SPD will defend an incumbent state premier - Torsten Albig - for the first time since Schulz's nomination as party leader.
"If Torsten Albig wins, whatever the coalition constellation, that will show people that the election in September is open," said Hajo Funke, political scientist at Berlin's Free University.
The SPD's prospects of toppling Merkel will be boosted further if they can hold onto power next Sunday in North Rhine-Westphalia (NRW), a large western region where elections in the past have served as an indicator of the national mood.
The left-leaning party has its work cut out.
Two polls on Thursday showed Merkel's conservative bloc extending its lead over the SPD at national level to six and seven percentage points respectively.
Jobs and wind farm plans are among the regional issues in Schleswig-Holstein, a state of 2.3 million voters that juts north of Hamburg and borders on Denmark, where Albig hopes to benefit from his incumbency advantage to return to power.
However, a survey by pollster Forschungsgruppe Wahlen for national broadcaster ZDF published on Thursday put support for the CDU at 32 percent in Schleswig-Holstein, ahead of the SPD on 29 percent.
That would leave a power-sharing "grand coalition" between the two big parties as the sole governing option for the rivals - a scenario that could also arise again at the federal level after the September election.
Writing by Paul Carrell; Editing by Angus macSwan