James Weinstein, founder of the progressive magazine “In These Times” and a historian of the American political left, died of brain cancer Thursday at his home in Chicago. He was 78.

Weinstein, who proudly called himself a socialist, forswore communism in 1956 after the truth about Stalin’s atrocities in the Soviet Union became public.

His biweekly magazine, decidedly nonprofit, was launched in Chicago in 1976, during the waning days of the New Left counterculture. It has managed to survive for a generation, attracting writers of liberal to radical sensibilities, long after the national culture turned to the right.

In several critically successful books, Weinstein argued that socialism was one of the neglected but true strains of red-white-and-blue in American politics and had contributed mightily to the success of the New Deal.

Studs Terkel, the Chicago writer and historian, Saturday compared Weinstein to such crusading journalists as George Seldes, Lincoln Steffens, and I.F. Stone, “a throwback to the muckraking journalists of the old days who questioned everything. In a sense, they represent what Tom Paine was all about.”

Weinstein first realized the cost of having a minority opinion in 1949, when he gave a friend of a friend a ride from the upstate town of Ithaca to New York City. The man, who “didn’t utter a word” during the entire four-hour trip, was Julius Rosenberg, who later would be executed in one of the most notorious espionage cases of the century.

Weinstein was never charged with a crime, but he was trailed and badgered by the FBI, which compiled a file of more than 2,000 pages on him. He also was called to testify before a grand jury investigating communist infiltration in the United States.

Born in New York City to a family that had made a fortune in real estate, Weinstein left Cornell University to serve in the Navy during World War II. He returned to New York after the war, resuming his studies and joining the Communist Party in 1948. He later described himself as a “Groucho Marxist,” or a leftist with a sense of humor. He attended Columbia University Law School but dropped out after a year. He worked for a year at the New York office of Young Progressives of America and then for six years as a tester and troubleshooter at several electronics companies, where he became active in two electrical workers unions. He returned to Columbia in 1956 for a master’s degree in history.

The next years took him to Madison, Wis., where he wrote two books on the Progressive era and edited a scholarly journal. A 1966 attempt at elected office, running as an independent candidate for Congress from Manhattan’s West Side, ended in a loss.

He moved to San Francisco, where he founded the Socialist Review and the Modern Times bookstore in the early 1970s.

After he started “In These Times,” Weinstein used its editorials to urge progressives to work within the Democratic Party and resist the tendency to splinter into “many little lefts.” He conceded that the impact of his journalism “has fallen far short of our initial hopes.” He retired from the newspaper in 1999.

The last of his five books, “The Long Detour: The History and Future of the American Left” (2003), earned praise from the Los Angeles Times because Weinstein “writes with an elegance and a light touch uncommon among historians of the left.”