German Chancellor Angela Merkel is set to form her fourth government after the opposition Social Democrats voted in favour of another grand coalition.
The vote by the more than 450,000 rank-and-file members ends five months of political deadlock since September’s inconclusive election.
The Social Democrats had been split between the party’s leadership, which backed joining the coalition, and its radical youth wing, which did not.
Volunteers began counting the postal ballots at 10 p.m. (2100 GMT) and worked through the night.
The SPD initially planned to go into opposition after a disastrous result in September’s election, but agreed to negotiate with Merkel’s conservatives after talks with the pro-business Free Democrats (FDP) and the environmentalist Greens on a three-way tie-up collapsed in November.
They thrashed out a coalition agreement which SPD leaders hailed for its commitments to strengthening the EU and giving them some key government roles. But opponents say the SPD risks irrelevance if it plays second fiddle to Merkel again.
Mrs Merkel has been in power for 12 years.