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BERLIN (Reuters) - Chancellor Angela Merkel’s conservatives thumped their Social Democrat rivals in a vote in the northern state of Schleswig-Holstein on Sunday, in a boost to her prospects of winning a national election in September.

Merkel’s Christian Democrats (CDU) fetched 33 percent of the vote in Schleswig-Holstein, up from 30.8 percent in the last election there in 2012, an exit poll for broadcaster ARD showed. The Social Democrats won 26 percent, down from 30.4 percent.

The result leaves the CDU short of sufficient support to rule alone in the state, but means the SPD cannot continue to govern in coalition with the Greens and the South Schleswig Party (SSW), which represents the ethnic Danish minority.

It also means the CDU has beaten the left-leaning Social Democratic Party (SPD) in two regional elections this year. The victory in Schleswig-Holstein was all the more significant because the SPD enjoyed the incumbency advantage.

A third state vote, in the large western state of North Rhine-Westphalia next Sunday, offers Merkel’s conservatives a chance to defeat the incumbent SPD again and build momentum in her bid to win a fourth term in office in September.