Aaron Copland was first introduced to the ideas of the left in 1919 through his friendship with Arne Vainio, who introduced him to the socialist newspaper The Call, and the political thinking of socialist Eugene Debs. Coupled with this was a deep admiration for writers like Frank Norris, Theodore Dreiser, and Upton Sinclair, who were creating novels that criticized the way that capitalism exploited the average person. Copland became a lifelong subscriber to The New Republic and The Nation, left wing magazines. Though Copland never formerly joined any political party, he was sympathetic to liberal causes. Pollack would write:

Copland never joined any political party. He described himself as "sympathetic toward the American-Liberal principles," saying, "If one likes people, is sympathetic to them and concerned about their welfare in general, one's personal leaning is in the direction of the democratic or liberal viewpoint." Although he avoided getting much more specific than this, one can postulate some of his guiding political ideals, including freedom of speech and thought, civil rights for all men and women, and social and economic justice for the common man. This last in particular overlapped with more purely professional concerns; the fight he waged on behalf of the American composer reflected, in its own way, the larger economic struggles of the working poor.

Most of Copland's friends were either radicals or left-of-center liberals. Some of his friends, like Clifford Odets, Paul Bowles, and Elia Kazan briefly joined the American Communist Party. The Young Composers' Group, which Copland belonged, was heavily influenced by leftist thought.

Copland didn't join the American Communist Party because of the restrictions that the Communists placed on artistic freedom, but he was deeply influenced by the push of American Communists to get artists to explore native folk music. During the mid 1930s, communists were threatened by the growing menace of fascism in Germany and Italy and Stalin decreed that communists were to have a truce with elements of the Left that they would normally be at war with. So communists made common cause with socialists and liberal reformers in fighting for social justice, fighting racial segregation in the South, and fighting to organize workers. Robert C. Cottrell wrote in his book Izzy: A Biography of I.F. Stone:

While it was clear, after a brief spell, that the New Deal was not ushering in a hoped-for revolution of the left or a feared one spearheaded by the right, it was also evident that the influence of progressive intellectual and activists on government policy was greater than ever. This development was possible not simply because the economic collapse had thoroughly discredited conservative panaceas, but also because many radicals had discarded their early insistence on the need for drastic change in the United States. And fearing the growing threat of fascist aggression, the Soviet Union- that ‘model’ socialist state- began, in the middle of the decade, to urge an antifascist alliance of liberal and radical forces. That development, coupled with the apparent New Deal successes at home, made peaceful reform appear increasingly attractive.

This temporary coalition of the radicals and liberals was called the Popular Front, and it had a big influenced on the arts of the 1930s.

Morris Dickstein wrote a wonderful book called Dancing in the Dark: A Cultural History of the Great Depression which talks of the social consciousness and concern for the average man that permeated artists as different as Frank Capra, Langston Hughes, and John Steinbeck. For Copland, this meant an exploration of American folk tales and historical figures for inspiration for his compositions. To reach out for a wider audience, he began composing for movies and for ballet.

Copland's political viewpoints also translated into participating in concerts for left wing causes and serving in committees. In 1945 Copland served as vice chairman of the National Council of American Soviet Friendship and chaired the American Soviet Music Society in 1946. After participating in the Cultural and Scientific Conference for World Peace in 1949, however, Copland felt he was manipulated by communists and he grew disillusioned by more radical left wing organizations. He remained a liberal, though, voting for Adlai Stevenson in 1952 and 1956 and John F. Kennedy in 1960s. Though he didn't approve of much of tactics of the radical leftists students of the 1960s, Copland opposed the Vietnam War, supported civil rights, and read writers like Herbert Marcuse, Kurt Vonnegut and H.Rap Brown.

Copland is an example of a liberal artist who made a significant contribution to the American culture. Nowadays, many conservatives have tried to demonize liberals as being unAmerican and have tried to monopolize patriotism as being the province of only the political Right. This ignores the fact that many of the cultural touchstones that conservatives laud as being American were created by artists who were to the Left of the political spectrum, Artists such as Norman Rockwell, Dr. Seuss, and John Steinbeck, for instance, were liberals. Thomas Hart Benton, who created the great American murals of the 1930s, was a former Marxist and a vociferous New Deal Democrat. Though Frank Capra was a Republican, the screenwriters to some of his best movies were liberals: Robert Riskin, who wrote It Happened One Night, Mr. Deeds Goes To Town, and Meet John Doe, was a New Deal Liberal; Sidney Buchman, the screenwriter to Mr. Smith Goes To Washington, was an American Communist. Dorothea Lange, Langston Hughes, John Steinbeck, Elia Kazan, Zora Neale Hurston, Charlie Chaplin, Katherine Hepburn, Stuart Davis, Reginald Marsh, and most of all, Aaron Copland were all leftists. This only goes to show that liberals love this country as much as conservatives, and have made a lasting contribution to the American scene.