Chancellor Angela Merkel's conservatives have suffered heavy losses in an election in Germany's most populous state, exit polls suggest.
Support for the Christian Democrats dropped from 35% to 26% in North Rhine-Westphalia, with the Social Democrats set to return to power with the Greens.
It is the Christian Democrats' worst result in the state.
Analysts say many voters rejected Mrs Merkel's tough line on fiscal discipline as a cure for state debt.
Voters in Greece, France and Italy also recently rejected austerity policies.
In another development, the exit polls suggested Germany's Pirate Party had won seats in North Rhine-Westphalia, making it their fourth state parliament.
The Pirate Party has grown in strength recently with its calls for transparency and internet freedom.
Nationally, Sunday's election will not change the balance of power, whatever the outcome but opposition leaders warn it may send an important signal ahead of national elections expected in late 2013.
Most populous state
According to two exit polls, the Social Democrats won around 38%, the Christian Democrats (CDU) 25.5%, the Greens 12%, the Free Democrats (FDP) 8.5%, the Pirates 7.5% and the Left, 2.5%.
The CDU and their national coalition partner, the FDP, recently lost elections in the northern state of Schleswig-Holstein. The CDU scored its lowest tally in the state for 50 years.
North Rhine-Westphalia, Germany's most populous state and with a large economy, has a history of influencing national politics.
The election was called in March after the state's minority government, run by a coalition of the SPD and Greens - narrowly failed to get a budget passed.
Polls had suggested state premier and SPD candidate Hannelore Kraft - who headed the fragile government for the past two years - would easily defeat CDU rival Norbert Roettgen, who is Mrs Merkel's environment minister.
In her campaign, Ms Kraft emphasised strengthening indebted local communities, investing in education and boosting the state's business appeal.
Mr Roettgen, on the other hand, accused the SPD of financial irresponsibility, holding rallies dominated by a huge inflatable "debt mountain", to emphasise the state's problems.
He provoked controversy early in the campaign by refusing to commit to being a full-time opposition leader if he lost. Such a move would cost him his job in Berlin.