In the early part of his political career, Debs was a member of the Democratic Party. He was elected as a Democrat to the Indiana General Assembly in 1884. After working with several smaller unions, including the Brotherhood of Locomotive Firemen, Debs helped to found the American Railway Union (ARU), the nation's first industrial union. When the ARU went on strike with the Pullman Palace Car Company over pay cuts, President Grover Cleveland used the United States Army to break the strike. As a leader of the ARU, Debs was sentenced to six months in prison for failing to obey an injunction against the strike. He was represented by Clarence Darrow at his trial.
Debs educated himself about socialism in prison and emerged to launch his career as the nation's most prominent socialist in the first decades of the 20th century. He ran as the Socialist Party's candidate for the presidency in 1900, 1904, 1908, 1912, and 1920, the last time from his prison cell in Atlanta.
In 1918 Debs was arrested for making a speech denouncing American participation in World War I. He was convicted under the Espionage Act of 1917 and sentenced to a term of 10 years. But President Warren G. Harding commuted his sentence to one of time served so that Debs could be released in time for Christmas of 1921. Harding did not issue a pardon. The White House released a statement saying this about Debs' case:
"There is no question of his guilt....He was by no means as rabid and outspoken in his expressions as many others, and but for his prominence and the resulting far-reaching effect of his words, very probably might not have received the sentence he did. He is an old man, not strong physically. He is a man of much personal charm and impressive personality, which qualifications make him a dangerous man calculated to mislead the unthinking and affording excuse for those with criminal intent."
According to historian David Pietrusza and others, Debs was loved and admired both by his fellow prisoners, as well as by the Warden and prison staff. Pietrusza describs Debs in saint-like terms. When Debs was released from the Atlanta Penitentiary, the other prisoners sent him off with "a roar of cheers" and a crowd of 50,000 greeted his return to Terre Haute to the accompaniment of band music. On the way home, Debs was warmly received at the White House by Harding, who greeted him by saying: "Well, I've heard so damned much about you, Mr. Debs, that I am now glad to meet you personally."
In 1924, Debs was nominated for the Nobel Peace Prize by the Finnish Socialist Karl H. Wiik on the grounds that "Debs started to work actively for peace during World War I, mainly because he considered the war to be in the interest of capitalism." But by the time of his release, Debs' health was not so good. In the fall of 1926, Debs was admitted to Lindlahr Sanitarium in Elmhurst, Illinois where he died of heart failure on October 20, 1926, at the age of 70.
A recent biography written about Debs in 2008 is Democracy's Prisoner: Eugene V. Debs, the Great War and the Right to Dissent by Ernest Freeberg. It sounds to me like advocates of free speech owe a lot to Debs.