By Stephen Brown
DUESSELDORF, Germany (Reuters) - Voting began on Sunday in Germany's most populous state where Angela Merkel's conservatives face a defeat that could give the left momentum before next year's federal election and fuel criticism of the chancellor's European austerity drive.
North Rhine-Westphalia (NRW), an industrial state in western Germany with an economy and population roughly the size of the Netherlands, has a history of influencing national politics.
First exit polls were due at 1600 GMT and expected to show Hannelore Kraft of the centre-left Social Democrats (SPD) trouncing her Christian Democrat (CDU) rival Norbert Roettgen, who is Merkel's environment minister.
"The SPD will get back in," said Helmut Krah, a voter in the NRW capital Duesseldorf who was window-shopping with his wife on the elegant Koenigsallee. "I'm voting for them not because they are good but because the others are so bad."
The vote is likely to bolster SPD fortunes nationwide and make Merkel, Germany's most popular politician, look politically vulnerable for the first time in a long while.
Kraft, who has run a fragile minority government with the Greens for two years, has won over voters by promising a go-slowly approach to cutting NRW's 180-billion-euro ($233 billion) debt, dodging accusations of fiscal mismanagement by the CDU.
A decisive victory for the SPD would be seen by many as a double defeat for the chancellor - NRW would be rejecting her party and the fiscal discipline she has forced on heavily indebted euro zone countries like Greece.
The vote in NRW follows elections in Greece, France and Italy that highlighted a growing backlash against austerity.
Roettgen has declared the vote a referendum on Merkel's European debt policies. "The election on Sunday also serves to finally give our chancellor full support from Duesseldorf ... for her national and European policies," he told a crowd of 2,000 at a campaign rally on Friday.
Other conservative leaders have distanced themselves from Roettgen and pollsters say a strong majority of Germans still support Merkel's insistence on fiscal discipline in Europe, even if they do not want as much austerity at home.
HARBINGER OF CHANGE?
NRW, home to one in six German voters, is a microcosm of Germany and changes in coalitions there have been harbingers of change for national governments. In 2005, the CDU led a centre-right coalition there to power four months before Merkel was elected with the same constellation in Berlin.
The latest poll released on Friday put the SPD on 38 percent and the Greens on 11 percent, well ahead of the CDU and their preferred Free Democrat (FDP) coalition allies.
The CDU is polling 33 percent, which would be the party's worst result in NRW, and the pro-business FDP look set to get just 5 percent of the vote. In the NRW election in 2010, the SPD were just behind the CDU.
"Hannelore Kraft set to win in NRW", "SPD-Greens on home stretch", read headlines at the weekend.
The upstart Pirates Party, whose platform is based on internet freedom and more direct participation in politics, look set to enter their fourth regional parliament in a row. They were polling at 8 percent.
Leaders of the FDP, junior partners in Merkel's centre-right coalition in Berlin, have sent signals that they might be open to a coalition with the SPD and the Greens.
"With the FDP and SPD, a performance-oriented party and a party focused mostly on social justice would rule together - surely it would be ideal if these two fundamental views came together," Wolfgang Kubicki, leader of the FDP in the northern state of Schleswig-Holstein, told Welt am Sonntag newspaper.
Were a similar coalition to come together at the national level in 2013, it could doom Merkel's hopes of a third term.
(Additional Reporting by Noah Barkin and Sarah Marsh in Berlin, Tom Kaeckenhoff in Duesseldorf; Editing by Mark Heinrich)