The centre-left Social Democrats and Greens – Germany’s main opposition parties – won combined support of about 51 per cent in the election in North Rhine-Westphalia state, according to exit polls and early counting.
That would be enough to give them a majority in the state legislature, which they narrowly missed in the last regional election two years ago.
Support for Mrs Merkel’s Christian Democrats was seen dropping to 26 per cent from more than 34 per cent, their worst showing in the state since the Second World War.
But the pro-market Free Democrats, Mrs Merkel’s struggling partners in the national government, performed respectably.
The incumbent government of popular governor Hannelore Kraft had been favoured to win, particularly after a much-criticised and sometimes gaffe-prone campaign by conservative challenger Norbert Roettgen, Mrs Merkel’s federal environment minister. “This is a crashing defeat for Mrs Merkel and her minister,” said Andrea Nahles, the Social Democrats’ general secretary.