cartwright served as the sdp's chief whip from 1983 onwards and as its president from 1987 until the failure of the continuing sdp in 1990. he also served as the sdp/liberal alliance's chief defence spokesman from 1983 to 1987. a close political ally of david owen, he stayed loyal to owen and owen's continuing sdp after the liberal party and a majority of the sdp merged in january 1988 to become the liberal democrats.