Clement, who was the long-time premier of North Rhine-Westphalia and for years one of the SPD's leading politicians, was reportedly taken completely by surprise by the decision. He had come under considerable fire from the party's left-wing for attacking the SPD and its leading candidate, Andrea Ypsilanti, during the state election in Hesse in January.
Clement can appeal the decision, however, the move is just one more sign of how much disarray there is in the Social Democratic ranks.
“The state arbitration committee's decision to eject Wolfgang Clement from the party has the potential to cause long-term damage to the Social Democrats,” said Clement's lawyer and former German Interior Minister Otto Schily, who added he intended to contest the matter on behalf of his client.
A centrist stalwart, Clement has repeatedly bemoaned the SPD's supposed drift to the left since the departure of former Chancellor Gerhard Schröder.
In January, Clement wrote a guest column for a conservative newspaper in which he implicitly warned against voting for the SPD in a crucial state election in the state of Hesse the following week. Left-winger Ypsilanti, he argued, would hobble the regional economy due to her vocal opposition to coal-fuelled power plants and nuclear energy.
Party leaders accused him of stabbing Ypsilanti in the back. His critics claimed that Clement, who serves on the supervisory board of German energy giant RWE, had exposed himself as a cynical turncoat.
"He called on people not to vote for our party and that is damaging behaviour," a local SPD leader in North Rhine-Westphalia, Rudolf Malzahn, told TV news channel N24, defending the party's decision. "He said he would do it again anytime. Regardless of what title and rank a person has - rules are rules."
Clement, 68, served as North Rhine-Westphalia state premier from 1998 to 2002 then as the "super-minister" for economy and labour under Schröder until 2005. During that period he incensed the left-wing of the party as one of the architects of former chancellor Gerhard Schröder's disputed Agenda 2010 economic programme.
But since leaving office, Clement frequently has criticized the SPD's moves to roll back the economic reforms he helped conceive under Schröder. Calls for his ouster multiplied and after the party disciplined him in April, it finally opted to expel him Thursday.
The SPD has lost considerable support under its current unpopular leader Kurt Beck and a stiff challenge from a hard-line socialist party called the Left made up of disgruntled Social Democrats and former East German communists. The SPD, Germany's oldest party, currently trails Angela Merkel's conservative Christian Democrats by 13 percentage points in opinion polls.