BERLIN – German Chancellor Angela Merkel braced for a bitter setback Sunday as voters in a state her party has ruled for half a century headed to the polls in an election clouded by events in Japan.

Merkel's conservatives have ruled Baden-Wuerttemberg in southwestern Germany since 1953, but anger over her nuclear policy as well as decisions on Libya and the euro suggest voters could turf her Christian Democrats (CDU) out of power.

A poll released Friday indicated a potential coalition of Social Democrats and the resurgent Greens were poised to unseat Merkel's CDU and their coalition partner, the pro-business Free Democrats (FDP).

The independent TNS Emnid institute survey showed the opposition could win 48 percent of the vote in a state that is home to industrial giants Daimler and Porsche, versus 43 percent for state premier Stefan Mappus's ruling alliance.

This would mark a debacle for Merkel, 56, after drubbings in North Rhine-Westphalia in May and Hamburg in February, with three more state polls to come this year.

"The election day that is making black-yellow tremble," ran a headline in mass circulation Bild am Sonntag, referring to the colours of Merkel's coalition.

Beyond a crushing blow to morale in Berlin, a defeat could make it even harder for Merkel to pass legislation in the Bundesrat upper house of parliament and prompt fresh calls for her to shore up her rightist credentials.

"Losing power in Baden-Wuerttemberg would be the worst defeat yet for Merkel as party leader," wrote the weekly Die Zeit.

Recent campaigning in the state has been dominated by the nuclear catastrophe in Japan, where officials Sunday discovered high radiation levels in water leaked from a stricken reactor at the Fukushima plant.

Calling Japan's crisis a "turning point", Merkel suspended for three months an earlier decision to extend the lifetime of Germany's nuclear reactors. Four of them are based in Baden-Wuerttemberg.

She also temporarily shut off the seven oldest reactors pending a safety review.

Nuclear power is unpopular in Germany, but polls indicated that voters saw Merkel's zigzagging as an electoral ploy: it has cost her support while boosting the anti-nuclear Green party.

An Emnid poll Sunday in the Bild am Sonntag suggested the Greens were flying high at a national level, gaining two percentage points to 20 percent.

The CDU was unchanged at 34 percent but the FDP fell to four percent.

"The CDU thought that a state which is doing so well could not be lost, but Mappus might just manage it," the centre-left daily Sueddeutsche Zeitung said.

"People's fear of nuclear power is at the core of Mappus' fear of defeat."

Also up for grabs Sunday is Rhineland-Palatinate, where the Social Democrats risk ceding their absolute majority and may form a coalition with the Greens.

The FDP, meanwhile, could fail to clear the five-percent hurdle for representation.

Adding to the pressure, tens of thousands of Germans hit the streets Saturday in four major cities to protest the government's nuclear policy. Organisers said as many as 250,000 took part.

In addition to the nuclear climb-down, conservatives have frowned on Berlin's abstention from a UN Security Council vote to create a no-fly zone in Libya, in a historic break with Western allies.

Critics saw the move as another sign of pandering, this time to a strong pacifist streak in the German electorate.

Finally, the media savaged Merkel for agreeing at an EU summit Thursday to commit to a giant new eurozone rescue fund.

"The old promise that 'we won't pay for the others' has been broken once again," the top-selling Bild newspaper wrote.

Some 10.8 million people are eligible to vote in the two states.

Polls opened at 0600 GMT and will close at 1600 GMT, with preliminary results due soon after.