On the advice of Stephen Lewis and Lloyd Stinson, Green joined the federal New Democratic Party (NDP) in the early 1960s soon after it was founded.
In the general election, Green finished well ahead of his three opponents, including Communist Party leader William Ross.
During this period, Russell Paulley, an old-style labour politician not popular among younger members of his caucus, led the provincial NDP.
One month later, the remainder of the provincial NDP caucus (aside from Ben Hanuschak, who backed Green) announced they would support Paulley in the leadership challenge, with the understanding he would stand down in favour of Ed Schreyer the following year.
The issue was subsequently resolved, and Green rejoined cabinet on July 21, 1972, returning to the portfolio of Mines, Resources and Environmental Management.
He became Minister responsible for the Manitoba Development Corporation on February 16, 1973, and remained in this position for the rest of the Schreyer government's lifetime.
He also publicly opposed an attempt by Russell Paulley (by then Labour Minister) to impose "back to work legislation" on striking Winnipeg Transit workers in 1976.
He said the party was becoming dominated by "the trade union movement and militant feminists", and opposed its plans to introduce anti-scab legislation if re-elected.
After Schreyer was appointed Governor General of Canada in 1979, Green stood for the party's interim leadership, but lost to Howard Pawley in a vote of caucus members.
[2] The Progressive Party was regarded as socialist initially, and supported traditional leftist causes such as full employment and increased profits taxation on resource industries.
In 1984, he contested a by-election in the south Winnipeg riding of Fort Garry, but finished fourth with 1,035 votes (Liberal leader Sharon Carstairs was also a candidate).
Green made further unsuccessful bids for the legislature in the 1990 general election in Kildonan, and the 1992 Crescentwood by-election, coming in fourth both times.
In a 1990 advertisement, the Progressive Party argued in support of balanced budgets and rejected any state sanctioning of "distinct status" for minority groups such as aboriginals and homosexuals.