BERLIN (Reuters) - The head of the coalition of Social Democrats (SPD) and Greens that runs the German state of Lower Saxony called for new elections on Friday after a deputy quit the Greens party, costing the alliance its one-seat majority.

German Chancellor and head of the Christian Democratic Union (CDU) Angela Merkel attends a news conference after meeting of their conservative bloc to discuss their election programme in Berlin, Germany, July 3, 2017. REUTERS/Axel Schmidt

The defection marks the latest in a succession of setbacks for the SPD as it prepares to contest a federal election on Sept. 24 in which Chancellor Angela Merkel is seeking a fourth term.

After running neck-and-neck with her conservatives early this year in opinion polls, the center-left party now trails by a double-digit score.

The departure of Elke Twesten to Merkel’s Christian Democrats (CDU) sparked anger among leaders of the SPD, which has already ceded power to the conservatives twice this year in state elections in Schleswig-Holstein and North Rhine-Westphalia.

Stephan Weil, leader of the Lower Saxony SPD-Greens government, said parliament in the rich agricultural state that also hosts major companies including Volkswagen should vote to dissolve itself.

“Voters have a chance to react now,” he told reporters. “That is why elections should be scheduled as soon as possible.”

Weil’s party said it expected the legislature to act on the recommendation in mid-August when it returns from summer break.

Weil had asked all parties in the legislature to meet on Monday for initial discussions about the crisis, a member of the pro-business Free Democratic Party told broadcaster ffn.

While the SPD-Greens could in theory continue as a minority government, CDU leaders also called for a new election, saying it should coincide with the federal ballot.

Twesten’s move, which Weil described as selfish, comes five months before scheduled elections in the state and leaves the SPD and Greens with 68 seats in the 137-seat assembly.

She said she had made the decision after the Greens declined to nominate her as a candidate in her district. “I see my political future with the CDU,” she said. “It wasn’t an easy step, but a necessary one.”

Long fierce adversaries at opposite ends of the political spectrum, the CDU and Greens govern together in several states including Baden-Wuerttemberg and Hesse.

Some polls suggest they could muster enough combined support on Sept 24 to form a governing coalition nationwide.

Meanwhile the SPD-Greens combination, which formed the federal government from 1998 to 2005, is losing ground.

Following this year’s regional elections and the Lower Saxony defection, it now heads viable administrations in only two of Germany’s 16 federal states: Bremen and Hamburg.