During the 1968 Democratic presidential primaries, Zampieri served as a delegate for Hubert Humphrey to the Democratic National Convention. He stated that if President Lyndon B. Johnson attempted to gain the presidential nomination at the convention then Zampieri would vote for him based on who his running mate would be although he stated that if Johnson was nominated then Richard Nixon would win the presidential election. All of the Humphrey and George McGovern delegates except for Zampieri and John Fitzpatrick performed a walkout after a proposed plank opposing the Vietnam War failed. He was one of Vermont's nine delegates to support Humphrey while seven other delegates supported Eugene McCarthy and six others supported McGovern.