Berlin (CNN) -- Chancellor Angela Merkel's ruling party took a beating in Germany's largest state Sunday in a vote its leading candidate called a "bitter" defeat.
Merkel's conservative Christian Democratic Union had claimed about 26% of the vote in the state of Northrhein-Westphalia, which includes the cities of Cologne, Bonn and Dusseldorf, according to returns aired by the German broadcasters ZDF and ARD. The center-left Social Democrats were leading with 39%, while their expected coalition partners the Greens were running at 11%.
The results are likely to be seen as a blow to Merkel's leadership, although the vote won't affect the balance of power in Germany's federal parliament.
Environment Minister Nortbert Roettgen, who had sought the state governorship on behalf of the Christian Democrats, took responsibility for what commentators were calling the worst loss for the party in the state's history.
"This defeat is bitter. It is clear and it really hurts," Roettgen told ARD. "I lost this election. It was my campaign, and my topics."
Northrhein-Westphalia is a longtime Social Democrat stronghold, and it was a 2005 win by the CDU that spurred the early elections that brought Merkel to office later that year.
Sunday's vote came a week after the CDU suffered losses in the northern state of Schleswig-Holstein, and after anger at German-backed European austerity policies contributed to the ouster of French President Nicolas Sarkozy and the Greek government.
But Josef Joffe, editor of the German weekly Die Zeit, said he expected Merkel to win a new mandate in the country's next federal elections, expected in 2013.
"There is no revolt the way you had a revolt on the left in France or the kind of revolt you had on the kind of neo-Nazi right and ultra left in Greece," Joffe told CNN's "Fareed Zakaria GPS."