BERLIN — Chancellor Angela Merkel’s party suffered a stinging defeat in elections in Germany’s most populous state on Sunday, one likely to embolden her political opponents both at home and abroad as the European debt crisis enters a critical new phase.

One week after Socialists seized the French presidency, the Social Democrats won the parliamentary election in North Rhine-Westphalia, early results and exit polls released Sunday showed. Norbert Röttgen, the lead candidate for Ms. Merkel’s Christian Democrats in the state, conceded defeat and announced that he would step down as the party head there.
Exit polls for German public television showed the Social Democrats winning 39.1 percent of the vote, an increase of 4.6 percentage points compared with the election two years earlier. While the results were not official, the party was likely to achieve a double-digit margin of victory. The Christian Democrats won just 26.3 percent of the vote, 8.3 percentage points less than in the previous election.
“This is a bitter day for us,” Mr. Röttgen told supporters. “We have suffered a clear and decisive defeat.”
In his concession speech, he took responsibility for the defeat, calling it “my loss” as a result of “my campaign, my themes,” but the ramifications went far beyond the borders of the state.
With the Green Party’s 11.4 percent of the vote, analysts say the state premier, Hannelore Kraft, of the Social Democrats, will be able to easily form a left-wing coalition to govern the state. For the past two years, Ms. Kraft had ruled with a minority government, forced to draw votes from the left or the right to pass each piece of legislation.
Mr. Röttgen ran against the debt-financed spending supported by Ms. Kraft, and even described the vote as a referendum on Ms. Merkel’s Europe policies.
Ms. Merkel and her party had hoped to sway the electorate with their hard line on Europe, painting Ms. Kraft almost as a domestic version of the spendthrifts the Germans complain about in Greece, Portugal and other economically struggling nations in Europe.
Instead, the voters handed Ms. Kraft, who pushed the state deeper into debt but hired more police officers and teachers and abolished fees for higher education, a significant victory. “We made people the central focus again,” Ms. Kraft told supporters on Sunday evening.
The German news media described the vote as a “debacle” and a “disaster” for the conservatives, one that also sends a clear signal that Ms. Merkel could face a difficult road to re-election. Peter Altmaier, the parliamentary leader of the Christian Democratic Union, called it “an extremely difficult day” for the party “as a whole,” a defeat that “surpasses our worst fears.”
With nearly 18 million inhabitants, North Rhine-Westphalia is home to more than one-fifth of the German population. A major loss here for the Social Democrats in 2005 helped pave the way for the defeat of Chancellor Gerhard Schröder, and Ms. Merkel’s own rise to that position.
“This will also strengthen us in Berlin,” Andrea Nahles, general secretary of the Social Democrats, said on German public television on Sunday.
The result vaults Ms. Kraft, 50, a down-to-earth candidate from the industrial Ruhr Valley, into the top rank of German politicians, prompting speculation that she might be the strongest contender to lead the party against Ms. Merkel and potentially succeed her as chancellor. “Queen of Hearts of the S.P.D.” read a headline on the Web site of the newspaper Die Welt, referring to her party by its German acronym.
Ms. Merkel’s party has performed poorly in numerous recent state elections. Just one week ago, the Christian Democrats were ousted from power in Germany’s northernmost state, Schleswig-Holstein. The next federal election is scheduled for September 2013, but the recent defeats have led commentators to speculate that Ms. Merkel could be forced into an early election.
Voters across Europe have expressed growing displeasure with Ms. Merkel’s path, punishing the mainstream parties in Greece that signed the country’s loan agreement with foreign creditors, which required deep spending cuts. In France’s presidential election, François Hollande defeated President Nicolas Sarkozy, Ms. Merkel’s close ally.
Mr. Hollande won in part by rejecting the German focus on austerity and promising more growth-oriented policies, as well as the renegotiation of the fiscal compact to rein in deficits signed in March by the leaders of 25 of the 27 European Union countries. Ms. Merkel has ruled out any renegotiation of the fiscal compact.
After his inauguration on Tuesday, Mr. Hollande will travel to Berlin to meet with Ms. Merkel to discuss the path forward for the Continent in crisis. The success of the Social Democrats could well give Mr. Hollande confidence in the difficult talks.
Even as the Christian Democrats tried to come to terms with the magnitude of their defeat, their coalition partners in Berlin, the pro-business Free Democrats, celebrated a remarkable turnaround. Before last week’s election in Schleswig-Holstein, the party was in the midst of a catastrophic slide, failing to reach the 5 percent threshold for representation in five out of the last six state elections. Analysts had begun to call the party’s very existence into question.
Building on a strong showing in Schleswig-Holstein, the party and its popular young leader in North Rhine-Westphalia, Christian Lindner, surpassed all expectations on Sunday, winning 8.6 percent of the vote. The surging Pirate Party won 7.8 percent of the votes, enough to enter the state Parliament for the fourth election in a row, helping the party firmly establish itself on the national political scene.