In 1968, AuCoin's opposition to the Vietnam War led him to co-chair Eugene McCarthy's Presidential campaign in Oregon's Washington County, west of Portland. AuCoin stayed with McCarthy after President Lyndon B. Johnson dropped out of the race. McCarthy's upset victory over Robert F. Kennedy in the Oregon Democratic primary encouraged AuCoin to run for elective office in 1970, seeking and winning an open seat in the Oregon House of Representatives in Washington County. Two years later, he was re-elected to the 57th Oregon Legislative Assembly. The Democrats took control of the chamber and he was elected House Majority Leader, the second highest position in the House.