* Merkel's conservatives win second regional vote this year
* They defeat incumbent Social Democrats rivals
* Result is boost for Merkel ahead of Sept. 24 national vote (Recasts with exit poll results)
BERLIN/KIEL, Germany, May 7 (Reuters) - German Chancellor Angela Merkel's conservatives won a clear victory over their Social Democrat rivals in a vote in the northern state of Schleswig-Holstein on Sunday, boosting 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.
"This is a clear mandate to become state premier of Schleswig Holstein," Jens Spahn, a senior member of the CDU, said of the state result.
The CDU's victory means it has now beaten the left-leaning Social Democratic Party (SPD) in two regional elections this year.
The 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 the western state of Saarland, where his party flopped in a March poll.
The result in Schleswig-Holstein, a region of 2.3 million voters that juts north of Hamburg and borders Denmark, is an even bigger blow for the Social Democrats because they have been the main ruling party there since 2012.
Should the SPD be unable to form a coalition government with other parties in Schleswig-Holstein, it will be the first time since 2012 the party has lost power in a state election.
The Greens won 13.5 percent of the vote in the state, the liberal Free Democratic Party (FDP) 11.5 percent and the SSW 3.5 percent. The anti-immigration Alternative for Germany (AfD), which has recently been losing support after a bout of infighting, scored only 5.5 percent.
A third regional vote, in the western state of North Rhine-Westphalia (NRW) 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.
SPD Secretary General Katarina Barley said the Schleswig-Holstein vote was a "bitter result" for the Social Democrats, but hoped her party would fare better in the next regional vote in NRW next Sunday and hold on to the state.
Merkel, who won the endorsement of former U.S. president Barack Obama on a farewell to Berlin visit last November, is presenting herself to voters as a crisis manager and Europe's anchor of stability.
Schulz has responded with a focus on fighting inequality, trying to sharpen the SPD's policy edge, which has been blunted by spending seven of the last 11 years sharing power as junior coalition partners with Merkel's CDU at the national level.
That arrangement turned voters off the two big parties and fed the rise of the AfD last year.
The FDP's candidate Wolfgang Kubicki said the results showed the SPD would have to broaden its election platform if it wanted to topple Merkel in the federal election in September.
"The Social Democrats have to become more innovative than just offering their slogan of fairness, fairness, fairness", he said. (Additional reporting by Michael Nienaber; Editing by Angus macSwan/Richard Lough/Susan Fenton)