Their decision put the Social Democrats and the Greens on the defensive, forcing them to deny any plans to strike an electoral pact with the Party of Democratic Socialism (PDS), the third biggest in the east.
"I have said it many times, and I shall say it again: there can be no co-operation or coalition with a post-Stalinist party," declared Joschka Fischer, the leader of the Greens. Mr Fischer was reacting to news that one of his MPs, Vera Lengsfeld, had crossed over to the government benches because she felt the Greens were "prepared to help the PDS back to power in order to get into power itself".
Ms Lengsfeld, a civil rights activist arrested in the 1980s by the Honecker regime, has strong personal reasons for opposing a party which has so far refused to repudiate its past. In 1992 she divorced her husband, after discovering that he had spied on her for the Stasi. The six other new Christian Democrats, not in parliament, were also known opponents of the Communist regime.
Until recently, the Greens and the Social Democrats have maintained a common front with the right against the PDS, convinced that the ex-Communists would eventually fade away. Their calculations have turned out to be wrong, and party strategists now expect the outcome of the next general election to hinge on votes in the east, and the mandates the PDS might be able to contribute to a putative "Red-Green" coalition.
The polls show that if elections were held today, the three parties together would in theory be able to dislodge Mr Kohl. But in practice, as the events of yesterday illustrated, even a hint of a deal with the PDS would exact retribution. Politics in the east continues to have a dynamism of its own, far removed from the fault lines which divide the rest of the country.