In July 1990 he joined the Social Democratic Party of Germany (SPD) and on 14 October of that year he was elected to the Brandenburg State Assembly. Shortly afterwards, on 1 November 1990, he was elected Ministerpräsident of the State of Brandenburg. He was re-elected twice and he held the position until his resignation on 26 June 2002. From May 1991 to 26 June 2002 he was a Member of the SPD National Executive. From 22 October 2002 to November 2005 Federal Minister of Transport, Building and Urban Affairs.