The party, then enjoying record popularity, had won control of the country’s richest state in March.

The Politbarometer poll published by ZDF TV found support for the Greens, which had risen to a peak of 23 percent in May, fell to 20 percent, falling 2 percentage points in the last month.

The dip comes at an awkward time for the Greens ahead of elections next month in two of Germany’s 16 federal states: Mecklenburg-Vorpommern on Sept. 4 and Berlin on Sept. 18.

The Greens had hoped to take control of Berlin after winning the state premier’s office in Baden-Wuerttemberg in March. But their support has been eroding in Berlin and nationwide.

The Social Democrats (SPD), the Greens’ preferred coalition partners, were unchanged at 29 percent in the ZDF poll and together the two center-left parties held a comfortable 49-38 margin over Chancellor Angela Merkel’s center-right coalition.

The next federal election is due in 2013.

Merkel’s ruling conservatives (CDU/CSU) gained 1 point in August to 34 percent in the ZDF poll. Their junior coalition partners, the Free Democrats, were unchanged at 4 percent and would thus fall under the 5 percent threshold to enter parliament if elections were held now.

Support for the Greens, which has long hovered at around 10 percent, had risen to near 20 percent in the last two years on fears of climate change and environmental issues before surging well above that level after the Japanese nuclear disaster.

The SPD has moved well ahead of the Greens in Berlin in the last two months after the two parties had been running neck-and-neck for much of the last year.

The Greens may have grown over-confident after taking control of Baden-Wuerttemberg in the spring with the SPD as their junior coalition partners for the first time. They were also leading the SPD in Berlin for a while in polls.

“We maybe got a little too intoxicated with our success after winning the election in Baden-Wuerttemberg,” an unnamed leader of the Greens told Die Zeit weekly newspaper on Thursday.

The SPD is set to retain its hold on the northeastern state of Mecklenburg-Vorpommern, Germany’s poorest state and the one with the country’s highest unemployment rate.

A total of 1,280 people were questioned for the Politbarometer survey conducted Aug. 9-11.