After serving as Askov's village clerk and mayor, Petersen won two terms in the
Minnesota House of Representatives, where he sponsored the state income-tax law and urged that tax revenues be spent on public education. Before he ran for the Minnesota Legislature he had been a member of the Republican Party. By the time he ran for office he was a member of the
Farmer-Labor Party. He served in the legislature from 1931 to 1934, representing the old House District 56. Petersen was elected the 28th
Lieutenant Governor of Minnesota in
1934 and served with Governor
Floyd B. Olson. He was sworn in as governor two days after Olson died of cancer on August 22, 1936. He served the remainder of Olson's term but declined to run for governor himself in the November general election, opting instead to launch a successful bid for Railroad and Warehouse Commissioner, a position he then assumed after leaving the governorship on January 4, 1937. Petersen was succeeded as governor by
Elmer Benson, a Farmer-Laborite from a more radical wing then Petersen. In 1938, Petersen ran against Benson in the primary. Petersen was unsuccessful in winning the nomination, but was successful in splitting the party along factional lines. He later ran for governor in
1940 and
1942, losing both times to
Harold Stassen. Despite being a supporter of
Franklin D. Roosevelt, Petersen opposed his bid for a third term in
1940, believing he should sabotage his own chances at winning by breaking the two-term precedent. ==Personal life==