German Chancellor Angela Merkel has told leaders of her party that she will not seek re-election as chairwoman at a conference in early December, a senior senior party source said. Her centre-left governing partners were on course for a dismal result, running neck-and-neck with the Greens for second place.
According to projections updated around 8pm, Ms. Merkel's CDU remained the strongest party in Hesse.
However, the SPD's historic losses could mean yet more pain for the chancellor's already troubled government coalition in Berlin.
The CDU has ruled Hesse for the last 19 years and just won the election at the weekend.
In last year's general election Ms Merkel saw a drop in support and was forced to form a coalition with the centre-left Social Democrats following an inconclusive result.
The CSU, led by German Interior Minister Horst Seehofer, lost 10 percent in comparison with the 2013 election and scored its worst result in 50 years, which was branded "painful" and "bitter" both by the CSU and CDU.
Andrea Nahles, the Social Democrats' leader, said that "the state of the government is unacceptable".
Political analysts in Germany have wondered whether Merkel might not serve out a full term if she's not leading her own party - a pairing that Merkel herself has said is crucial to being an effective chancellor. Furthermore, 63 percent of respondents said that the SPD should exit the grand coalition in Berlin and seek renewal in the opposition.
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"She will not stand again for the chairmanship of her party", said the source within the Christian Democratic Union, a day after voters in the Hesse state punished her party in a regional poll.
The other big victor was the far-right Alternative for Germany (AfD), which entered the Hesse regional assembly for the first time with 12.8 percent of the vote, the ZDF projection showed.
Voters have appeared generally satisfied with Bouffier's outgoing state government.
She intends to remain as chancellor until 2021, until the parliamentary term runs out, the Associated Press cited her saying.
The Social Democrats only reluctantly entered Merkel's fourth-term national government in March, and many are dismayed by what has happened since.
This will distract her from tending to major challenges at home and overseas - ranging from overcoming a digital deficit and pushing the German auto industry toward cleaner mobility to seeing through euro zone reforms and managing Britain's planned departure from the European Union.
For a start, Mrs Merkel's coalition...
After 18 years as party chief, the move throws open the succession race for a post that has traditionally has been the springboard for CDU chancellors.