Forsey was a supporter of the Conservative Party led by Arthur Meighen until he went to Balliol College, Oxford, on a Rhodes Scholarship during which he was converted to democratic socialism.

Upon returning to Canada, he joined the League for Social Reconstruction, and was a delegate at the founding convention of the Cooperative Commonwealth Federation (CCF) in 1933 in Regina.

This was an old farmhouse and property that the Masseys had bought in 1918 on rising land backed by rolling hills and facing Lake Ontario a few miles to the south.

While he had become a social progressive, he remained a "Constitutional conservative", and wrote his PhD thesis on the King–Byng Affair, defending the positions of Arthur Meighen and Governor-General Lord Byng of Vimy.

In 1958, Forsey, though still a CCF member, was appointed by the Progressive Conservative government of John Diefenbaker to the Board of Broadcast Governors, the predecessor of the Canadian Radio-television and Telecommunications Commission.

Later in the 1960s, he was attracted to the views of Pierre Trudeau on the Canadian constitution, and joined the Liberal Party of Canada upon being appointed to the Senate in 1970.

Under Prime Minister Louis St-Laurent, compromises were reached that quietly, and without legislation, "dominion" was retired in official government names, titles and statements, usually replaced by "federal".

In his many commentaries on constitutional issues, especially the reserve powers of the Crown, Forsey was a conspicuous supporter of the action of the Governor-General of Australia, Sir John Kerr, in dismissing the Prime Minister, Gough Whitlam, in the 1975 constitutional crisis because his government was unable to obtain supply (approval to spend money) from the parliament and refused to call a general election.