Voters in Hesse, a large, prosperous state in central Germany, flocked to the Christian Democrats, who had been out of the regional government since 1991. According to provisional results, the Christian Democrats gained 43 per cent of votes, about 4 per cent more than Chancellor Schroder's Social Democrats.
With the Green vote collapsing, the "Red-Green" coalition lost not only its majority in the Hesse assembly, but also, by consequence, in the upper chamber of the federal parliament, the Bundesrat.
As a result, Mr Schroder is confronted with the sort of legislative impasse that paralysed Helmut Kohl's administration in its final years.
Although Social Democrat leaders sought to present the debacle as a little local difficulty, there is no doubt Hesse's 4.3 million voters were expressing a verdict on the governing coalition in Bonn. The regional government of Hans Eichel, a veteran Social Democrat politician, was popular.
Like Mr Schroder's administration, the outgoing government of Hesse consisted of Social Democrats and Greens. The region has a strong Green party that grew out of the Sixties revolt and subsequent anti-nuclear protests. It was in Hesse that Germany's Greens first entered office, in 1985, when a Frankfurt activist named Joschka Fischer took the oath as environment minister, wearing a sports jacket and trainers.
Many Greens disapprove of Mr Fischer's performance as Foreign Minister in Bonn, blaming him for caving in to Mr Schroder's Social Democrats over the pace of nuclear-plant closures. The Greens' share of the vote fell from 11 per cent in elections four years ago to less than 7 per cent yesterday.
The most important issue, though, had nothing to do with the performance of Greens, local or otherwise. Two-thirds of those polled said they were opposed to the new nationality law the Bonn government was proposing to ease the integration of foreigners.
Breaking with their middle-of-the-road traditions, the Christian Democrats took their protest against the law into the streets. Their petition, launched in Hesse, netted more than half a million signatures in recent weeks. As conservative politicians campaigned with slogans such as "We don't want Chinatowns in Germany", voters were invited to sign up to an essentially xenophobic plebiscite.
The CDU's allies, the Free Democrats, expressed their disgust at the petition and nearly lost their seats in the regional assembly as a result.
Despite the outcry, right-wing Christian Democrats felt vindicated. "If this result stays, we shall use our new majority in the Bundesrat to scupper the Social Democrats' plans," Angela Merkel, the CDU's secretary, said last night.
Manfred Kanther, a federal interior minister under Mr Kohl, said: "This is a clear majority against dual citizenship." Roland Koch, the Christian Democrats' governor-designate for Hesse, said: "This result definitely has to do with the bad government policy in Bonn."
Most pundits agreed that the citizenship law was a key factor shaping the election, although high unemployment in rural parts of Hesse was also an issue.