As leader of the Communist Party of Sweden, Hagberg upheld a clearly pro-Soviet line. He supported the Soviet crushing of the Hungarian Revolution of 1956 and he also defended the building of the Berlin Wall as "serving the cause of peace".[1]
In the municipal elections in 1962, the Communist Party hit a record low of 3.8% of the votes, and critics within the party blamed this on the party's pro-Soviet line. In 1964, Hagberg was replaced as party chairman by C.-H. Hermansson. When a pro-Soviet faction broke away from the party in 1977 to form the Workers' Party - the Communists, Hagberg joined them.
Hagberg died in his home in Luleå on 17 December 1993. His memoirs, titled Jag är och förblir kommunist ("I am and will remain a communist"), were published in 1995.