Meckel had already been involved with domestic opposition to the East German Communist regime since the 1970s.

In October 1989, he initiated the establishment of the Social Democratic Party in the GDR (SDP) together with Martin Gutzeit.

From 23 February 1990 up to the merger with the West German SPD on 27 September 1990, he was deputy SDP party chairman.

In his term of office, he frequently worked with Hans Dietrich Genscher, as he was one of the representatives of the two German states during the Two Plus Four Negotiations with the victors of the Second World War over a Final Peace Treaty.

From 1992 to 1994 he was spokesperson of the SPD parliamentary group in the historical commission on the aftermath of the SED dictatorship in Germany.