Caplan has been a political activist all his life as a member of the CCF and its successor, the NDP. He was campaign manager in a series of election campaigns, both federal and provincial, including those for Ontario leader
Donald C. MacDonald and national party leader
David Lewis. A lifelong friend of David's son
Stephen Lewis, he was also a close advisor and senior strategist to Lewis prior to and during his career as leader of the
Ontario New Democratic Party in the 1970s. After a period serving as co-ordinator of Toronto's health advocacy unit from 1980 to 1982, Caplan returned to political work. From 1982 to 1984, Caplan was federal secretary of the New Democratic Party, and was national campaign manager for the
1984 general election. In 1982, Caplan was a candidate for the federal NDP nomination in
Broadview—Greenwood after
Bob Rae vacated the seat to move to provincial politics. Caplan was upset in his bid for the nomination by
Lynn McDonald, who defeated Caplan on the third ballot by a margin of 309 votes to 248. McDonald went on to keep the seat for the NDP in the ensuing
by-election. ==Broadcasting==