Though the general picture of American Communism is of an assorted cast of unimportant radicals who never threatened anything, the truth is quite different and this archived 2004 article from Claremont Magazine explains.

An excerpt.

From about 1955 to 1995, the dominant opinion in the United States held that the American Communist Party (CPUSA), founded in 1919 in the wake of the Communist revolution in Russia, was a small collection of admirers of the Soviet Union that never amounted to much. In the 1930s (so the story went) they mobilized a number of “popular fronts” to oppose fascism and promote various leftist causes. In the 1940s, a few Communists—probably Julius Rosenberg and (arguably) Alger Hiss—went so far as to commit acts of espionage on behalf of the Soviet Union. But Rosenberg was executed, and Hiss went to prison; so why all the fuss about domestic Communism?

Far worse than such rare cases of misplaced loyalty (in this view) was the damage wrought by opportunistic politicians who seized on the existence and supposed misdeeds of the CPUSA to alarm American public opinion and ruin the reputations of innocent liberals. One of the earliest such persecutors was Congressman Martin Dies, a Texas Democrat who in 1937 persuaded the House of Representatives to create a Special Committee on Un-American Activities, which became a standing committee in 1945 and lasted for 30 years, hounding Hollywood actors and many other victims.

But by far the greatest villain among Red-hunting politicians was, of course, Wisconsin’s Republican senator Joseph McCarthy, who raised the issue of Communists in government in February 1950 and rode it triumphantly for four-and-a-half-years, acquiring an immense popular following, until the Senate itself voted to “censure” him in December 1954. He died, of liver failure induced by alcoholism, in May 1957, at the age of 48. By the 1960s the CPUSA, reduced to a few thousand members, had been almost wholly superseded by the New Left, and barely survived to see the collapse of the Soviet Union in 1991.

That was the story of American Communism and its foes, as successfully propagated by the nation’s dominant liberals, and it remained, as we have noted, the conventional wisdom for forty years. Indeed, it is in some ways the conventional wisdom even today, for younger generations (including many conservatives) have never heard any other version of the facts.

But the year 1995 was an epochal one for the study of American Communism. For in that year, thanks to the insistence of the late Democratic Senator Daniel Patrick Moynihan of New York, who had long specialized in intelligence matters, some 2,900 documents collectively known as “the Venona papers” (a deliberately meaningless code phrase) were de-classified and published. These were radio messages from the top KGB agents in Washington and New York to their superiors in Moscow from approximately 1943 to 1948. They had been recorded at the time by the U.S. Army Signal Corps, but they were, of course, in code, and their decoding was an immensely arduous job carried out by a number of heroic government cryptanalysts over the period from 1945 to 1980.

A second new source of information on the American Communist Party was the archives in Moscow of the defunct Soviet Union, which began to be partially accessible to American investigators in the early 1990s, during the Yeltsin years.

The Venona papers, together with these archives, made it absolutely clear that the American Communist Party was from its beginning the willing agent of Soviet intelligence, obedient to its orders, financed by its contributions, and serving not only as a propaganda organ for Soviet policies but as a generous source for the recruitment of agents who would thereupon influence American policy and gladly commit espionage as well. It is now plain that by 1945 every important branch of the American government, from the White House itself to the State Department, the Defense Department, the Justice Department, the Treasury Department, the Office of Strategic Services (predecessor to the CIA), and the Office of War Information, to name only a few, was infested with Communists busily doing the work of the Soviet Union.

Moreover, it is obvious that a penetration so complete would have been impossible if the Communists had not been able to depend on the blindness or indifference of many of the far larger number of ordinary liberals who dominated the Roosevelt Administration. As early as the late 1930s, even known Communists in government were often regarded by their colleagues as merely “liberals in a hurry.” And during the war, of course, they could be excused as simply enthusiasts for America’s doughty ally, “good old Joe.”