The opposition alliance has narrowly beaten German Chancellor Angela Merkel's centre-right coalition in regional elections in Lower Saxony.
The Social Democrats and the Greens won a single-seat majority in the state legislature, a region of eight million people in north-western Germany.
The election is seen as a bellwether for national elections in September and the Lower Saxony defeat has set alarm bells ringing for Chancellor Merkel, the BBC reports.
The results show a fall in support for the Christian Democrats, despite her prominence as a campaigner for them
Mrs Merkel's CDU coalition has lost a number of state elections as she seeks a third term as leader of the country.
Sunday night's knife-edge finish saw the SPD and Greens winning a combined 46.3% of the vote to the centre right's 45.9%.
If these results were replicated in the federal elections later this year, Chancellor Merkel's government in Berlin would be threatened, the BBC reports.
She remains the most likely leader of Germany after the national elections, but can't count on it and looks to have a fight on her hands.
