BERLIN — Chancellor Angela Merkel’s main rivals pulled off an upset in a regional election, eking out a one-seat majority that will usher the opposition Social Democrats into power in the state of Lower Saxony, months ahead of balloting for the national Parliament.
Preliminary results released on Monday showed the center-left bloc of the Social Democrats and the Greens securing a one-seat majority in the regional legislature in Hanover.
The shift signaled an end to a decade of center-right government in the state and will have a direct impact at the national level by tipping the balance of power in the upper house of Parliament, the Bundesrat.
The victory in Lower Saxony gives the center-left a majority in the Bundesrat that could allow it to block legislation from the lower house, dominated by Ms. Merkel’s center-right bloc.
“I expect that it will hardly be possible to push through proposals that the S.P.D. opposes,” Volker Kauder, the parliamentary leader for Ms. Merkel’s party told ZDF public television on Monday. “We will have to see if they use it.”
The Social Democrats took 32.6 percent of the vote, while the Greens won 13.7 percent, the preliminary results showed, giving them 69 seats in the regional legislature. Although the Christian Democrats emerged as the strongest party with 36 percent of the vote, combined with their Free Democrat partners, they were able to secure only 68 seats.
The Free Democrats, the junior partner in Ms. Merkel’s governing coalition in Berlin, won 9.9 percent of the vote.
Neither the Pirate Party nor the Left Party cleared the 5 percent hurdle need to secure representation in the Lower Saxony legislature.
It was unclear how much the loss will hurt Ms. Merkel, who enjoys overwhelming popularity in Germany, thanks to a relatively robust economy, low unemployment and her hard-nosed handling of Europe’s debt crisis.
Ms. Merkel made seven appearances in Lower Saxony this month, alongside the state’s governor, David McAllister.
Although he was the incumbent, the campaign was the first for Mr. McAllister, 42, who took over the position in 2010 when his predecessor, Christian Wulff, was called to Berlin to become president. The son of a soldier from Scotland and a German mother, Mr. McAllister insisted throughout the campaign that his tenure had brought prosperity to the region, and he urged voters to support continuity.
He was expected in Berlin for talks at the Christian Democratic headquarters later Monday.
His main challenger, the mayor of Hanover, Stephan Weil, who ran for the Social Democrats, struggled to make himself better known among voters, resorting at one point in the campaign to handing out red roses to prospective voters, which stretches across largely rural countryside from the North Sea to the former inner-German border.
“I am excited about five years of red-green,” Mr. Weil told reporters after realizing his bloc’s success, using the traditional color codes for his party and their partners. “That was a real roller-coaster ride tonight.”
Although local issues tend to dominate regional elections — as was the case in Lower Saxony, where the education system and completion of several infrastructure projects dominated the debate — the outcome could help the Free Democrats improve their image at the national level of a party dogged by a leadership crisis.
Many had blamed the party’s chairman, Philipp Rösler, who also serves as economy minister and consistently ranks among the country’s least popular politicians, for failing to focus on concrete issues.
