 Between 1981 and 1990, Finkelstein was a member of the SDP, becoming Chair of the Young Social Democrats on the defection of his predecessor Keith Toussaint to the Conservative Party during the 1983 general election campaign.Subsequently he was elected youth representative on its National Committee, selected as a parliamentary candidate (Brent East 1987) and became a close ally and adviser to David Owen, the SDP leader.When the merger with the Liberal Party was proposed, Finkelstein was among the leading opponents and refused to join the merged party.