The extraordinary comments by Edmund Stoiber, the right-wing Prime Minister of Bavaria, were seen as an insult to Angela Merkel, the conservative party leader and east German who is hoping to become the country's first female Chancellor after the September poll.
Mr Stoiber's criticism was aimed at the 33 per cent of east German voters who have thrown their support behind a new radical "Left Party" which is currently the strongest political force in the region.
"Have you all gone mad?" Mr Stoiber asked voters at campaign rallies in the east. "Only the most stupid of calves vote for their own butcher," he added. His remarks followed similar attacks last week when the Bavarian leader claimed that east Germans voters were "frustrated" and "not as clever" as his native Bavarians.
Mr Stoiber, who has been nicknamed the "Bavarian Pit Bull" for his aggressive campaign style, led Germany's conservatives to defeat in the country's 2002 general election. He refused to apologise for his comments yesterday despite being rebuked by Mrs Merkel and several other senior conservatives.
"I have told him that east and west can only go forward together. Anything else is counter-productive," Mrs Merkel said. "We have only got five weeks until the election and we have to clear up the damage," added Jörg Schönbohm, the conservative leader in the east German state of Brandenburg. "Stoiber should think about what he is doing," he said.
The row seemed destined further to undermine Mrs Merkel's hopes of a certain election victory. Recent opinion polls have suggested that her conservatives can no longer bank on gaining an absolute majority during the election as had been the case throughout June and July.
Her party has adopted the Rolling Stones' hit song "Angie" in a bid to bolster her popularity. But weak television performances and her failure adequately to explain how she intends to tackle Germany's 5 million unemployment problem have prompted commentators to claim she does not deserve to become the next Chancellor.
Mr Stoiber's remarks and his refusal to apologise for them underlined conservative unease about Mrs Merkel's suitability as a candidate for Chancellor. Germany's main television channel noted at the weekend that the conservative prime ministers of several key states were conspicuously absent at Mrs Merkel's campaign rallies in their regions last week.
There were also reports yesterday of an angry shouting match between Mrs Merkel's conservatives and Mr Stoiber over party campaign strategy. Mr Stoiber's Bavarian conservatives were said to have accused Mrs Merkel's supporters of running a "sloppy" campaign.
Equally revealing were remarks by Hugo Müller-Vogg, Mrs Merkel's biographer, who disclosed last week that there was a strong element within Germany's predominantly Catholic conservatives which remained opposed to the idea of an east German female Protestant becoming the next conservative Chancellor.
"There are those who say: if Merkel wins it will be good - if she loses it will also be good," Mr Müller-Vogg said in an interview.
By contrast, Chancellor Gerhard Schröder tried to add to his Social Democrats' gradually increasing popularity at the weekend. Echoing the anti-Iraq war strategy that won him Germany's 2002 election, he warned President Bush against going to war against Iran.