A quick-witted bodyguard just managed to open an umbrella to prevent rotten eggs splattering Roland Koch, the prime minister of Hesse. The crowd in Frankfurt’s sedate Opera Square was in uproar.

“Listen,” said the 49-year-old conservative. “You have to be able to talk about subjects like this in a city where 66 per cent of young people have an immigrant background!” More boos.

Suddenly German politics is beginning to look turbulent. Regional elections are to be held tomorrow in the large states of Hesse and Lower Saxony, and in Hesse, at least, a clear division is emerging between an angry, increasingly radical Left and a disorientated, conservative Right.

Despite, or because of, the antiimmigrant campaign Mr Koch could well be toppled — and accelerate the end of Angela Merkel’s coalition Government. The prospect of defeat is certainly rattling Mrs Merkel. In Frankfurt, her voice grew shrill as, for the first time in her tenure, she had to put up with public booing. “All this yelling shows what is at stake on Sunday,” she said. Voters had to choose, she said, between Mr Koch — “the architect of the Christian Democrats’ pro-business policies” — and a broad alliance of backward-looking leftist parties.

A few years ago Mrs Merkel would have been shedding, at best, crocodile tears for Roland Koch. He was part of a group of Christian Democrat alpha males who, during a political freebie to South America in 1979, vowed never to run against each other. Others on the trip included Christian Wulff, who became prime minister of Lower Saxony, and Günther Oettinger, prime minister of Baden-Württemberg. They formed the secret opposition to Mrs Merkel — an outsider because of her east German, non-Catholic background — but each has since made his peace with her. Until she stumbles.

Now it seems that it is Mr Koch who may stumble. The anti- immigrant campaign looks as if it will backfire: he is only just level with his challenger, the Social Democrat Andrea Ypsilanti, and seems certain to lose his absolute majority. Mr Koch is a hard man. When a retired teacher was beaten up by two immigrant youths after asking them to stop smoking on the subway, Mr Koch called for young offenders to be sent to boot camps and for immigrant criminals to be deported. It was unacceptable, he said, that Turks flouted German values by slaughtering animals in their kitchens.

Mr Koch swept to power on the back of a similar tactic in 1998 but this time the rhetoric is beginning to jar. This is partly because the Turkish community is more accepted and partly because Mr Koch has a patchy record on juvenile offenders, but mainly because he is no longer the whizz-kid local hero, likely to succeed Mrs Merkel as Chancellor. This weekend could mark the beginning of the end of the career of Germany’s only red-blooded tooth-and-claw politician.

If Koch-conservatism is discredited for flirting too obviously with far-right xenophobia, Mrs Merkel will have to seek a new Christian Democratic identity. The Social Democrats have moved to the left without abandoning the centre ground. Robbed of Roland Koch, she cannot move her party to the right.

The most popular politician in Europe is looking vulnerable. The eggs may have missed her but the boos are still echoing in her ears.

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