Rensenbrink's first foray into politics was a letter-to-the-editor at the age of 14 praising Minnesota's political leader
Harold Stassen. The letter appeared in the Minneapolis Star Journal. It was the first of many letters to the editor in that newspaper during the next several years. While in college Rensenbrink participated in a popular campaign to unseat the mayor of Grand Rapids. Shortly after, at the University of Michigan in 1951–52, he joined the
Young Republicans, but found himself disgusted with the politics of
Joseph McCarthy. Rensenbrink left the Republican Party and became a
Democrat after listening to the speeches of
Adlai Stevenson, the Democratic nominee for president in 1952 and 1956. In 1968, following the assassinations of
Martin Luther King Jr. and
Robert F. Kennedy, and following Chicago Democratic
Mayor Daley's police crackdown of student protestors at the national Democratic nominating convention in Chicago, Rensenbrink formed with others the Reform Democrats of Maine (1968 to 1970). The attempt was to help end the Vietnam War and to reform the Democratic Party. In 1976 and 1978, Rensenbrink ran in the Democratic primary for the Lewiston/Auburn/Topsham Senate seat of the
Maine legislature. The district was heavily Democratic so a victory in the primary all-but ensured victory in the general election. He came up short both times, losing by only 170 votes in 1978. During the early 1980s, Rensenbrink joined with others in campaigns to close Maine's only nuclear plant. The campaigns were battles lost, by close margins, but the struggle against nuclear power was won in terms of public opinion. The nuclear plant, "Maine Yankee," closed down within several years of these campaigns.
The Green Party 1984 was a decisive year for Rensenbrink. While in Poland the previous year, he had heard about the German elections where the
German Green Party participated. They had won
27 seats in the German Parliament. That summer, on his way back to the United States, Rensenbrink stopped off in Munich and in Frankfurt to visit friends who had joined the German Green Party and were celebrating their unexpected parliamentary success. That fall, back in Maine and Bowdoin College, Alan Philbrook, a fellow anti-nuclear activist, called to say he had been at the first meeting of the Greens in Canada and, on his return to Maine, registered the Green Party of Maine. The two then called a meeting for January 14, 1984, to consider forming a Green Party organization in Maine. This was accomplished in Augusta on that date—the first of its kind, as they later discovered, in the United States. Rensenbrink quickly made plans to seek early retirement (which was accomplished in 1989) and threw himself into Green Party organizing in Maine and in the United States. A meeting in
St. Paul, Minnesota was followed by regular monthly meetings of what was first called the Committees of Correspondence, later changed to Green Committees of Correspondence, with a Clearing House in Kansas City headed by Dee Berry. Working with the Clearing House, the annual gathering of a Green Assembly, and the Inter-Regional Committee that had formed, Rensenbrink headed a three-year project to produce a Green Policy Program, generated from the over 300 grass roots groups that had sprung up in the early years. The Program was completed and approved in September 1990, at the annual meeting of the Green Assembly in Boulder, Colorado. Thereafter, Rensenbrink, with others, formed the Green Politics Network whose aim was the eventual creation of a national Green Party of associated state Green Parties. The result was the Association of State Green Parties (ASGP), which, from 1996 to 2001 grew to include all of the state Green parties and then morphed into the
Green Party of the United States in 2001. Rensenbrink has continued to be active in the USGP's National Committee and annual Conventions and Presidential campaigns and in its International Committee, which he had founded in 1997 as part of ASGP. Rensenbrink was the Maine Green Party's candidate for U.S. Senator in
1996 against Republican
Susan Collins and Democrat
Joe Brennan. and received 4% of the vote. The Maine Green Party changed its name to
Maine Green Independent Party (MGIP) in 1998. It has grown steadily. With more than 41,000 Maine residents enrolled in the party, it has the largest per capita membership of any Green Party in the United States. It fielded a candidate for governor in each four-year election cycle from
1994 to
2006, getting 10% of the vote in
2002 and 2006. Rensenbrink worked as campaign manager for two gubernatorial campaigns:
Jonathan Carter in 1994 and
Pat LaMarche in
1998 and worked as a major advisor in the others. MGIP has run 10 or more candidates for the state legislature in most elections, placing one in the state
House of Representatives in 2002 and 2004. It has become the second party after the Democrats in Maine's largest city, Portland, and, for several years, has had three of its members in the Portland City Council. It has also elected several members to the School Board. ==Community work==