In 1946, Blatnik was elected to Congress representing Minnesota's 8th District in the northeastern part of the state, running on the newly unified ticket of the Minnesota Democratic-Farmer-Labor Party. He was reelected 13 times without much difficulty. He served in the 80th, 81st, 82nd, 83rd, 84th, 85th, 86th, 87th, 88th, 89th, 90th, 91st, 92nd, and 93rd congresses, (January 3, 1947 – December 31, 1974).
In 1963, Blatnik, introduced a bill to make Leif Erikson Day a nationwide observance. The following year Congress adopted this unanimously.
Upon retirement, Blatnik endorsed his long-time administrative assistant, Jim Oberstar, to replace him in Congress; Oberstar won easily in the 1974 election.
Personal life and death
In 1955, Blatnik married the former Gisela Hager. They had three children. Blatnik died in Forest Heights, Maryland on December 17, 1991. He was survived by his second wife, the former Evelyn Castiglioni.