In 1937 he resigned from the Times to join the staff of Time magazine as radio critic, a position that he in turn resigned to join the staff of Prime Minister, a left-of-center New York City daily. In the postwar period, McManus was actively involved in electoral politics. In 1948, he served on the national committee of the Progressive Party in support of the presidential candidacy of former vice-president Henry A. Wallace.
In 1950 and 1954, McManus ran for Governor of New York on the American Labor Party ticket.
In 1949, McManus co-founded the National Guardian, a progressive newspaper, with fellow former Times writer James Aronson. The paper was critical of the Cold War and McCarthyism and supportive of the labor movement and racial equality.
He would co-edit the paper until his death. In November 1955, McManus and Aronson were among 26 former and current New York Times employees subpoenaed by the Senate Internal Security Subcommittee.
The subcommittee was investigating Communist infiltration in the American media.
The subpoenas were based upon the testimony of Winston Burdett, a famous Columbia Broadcasting System war correspondent. In 1956, McManus testified, citing Fifth Amendment protections to avoid naming names or admit any knowledge of Communist activities.