Robert LaFollette, Senator from Wisconsin, 1906-25.
Fighting Bob LaFollette was perhaps the prototypical Progressive senator. Deeply concerned with the changes rapid industrialization and urbanization had caused the United States, no one worked harder to make life decent for Americans in the early twentieth century than LaFollette.
When LaFollette entered the Senate in 1906, he came into a body controlled by corporate interests. We often see 1901 as this hard line between the Gilded Age and the Progressive Era, but we all know how seniority works in the U.S. Senate--all the committee heads in 1906 had long tenures during the most corrupt era in American history, opposed many Progressive reforms, and liked the good old days of bribery that they profited off of. So when LaFollette entered, he was a nobody, exiled to marginal committees to limit his influence. But he wasn't a good boy--he was loud and toured around the nation exposing corruption within Congress and rallying for progressive causes. Because of this, he quickly became a leader of the Progressive movement in the Senate, giving him much more power than his lack of seniority normally would have allowed.
LaFollette worked for any number of issues, mostly surrounding improving conditions for the working class. He opposed child labor and supported women's suffrage. He was an early proponent of social security, laying the groundwork for Roosevelt to pass that legislation in the 1930s. He actively opposed U.S. imperialism in Latin America at a time when few cared. He believed in government ownership of utilities, assisting people like Nebraska senator George Norris on issues like government-owned power that would bring the 20th century to rural Americans, as well as pro-labor laws that workers would eventually see help them in the 1930s.
His steadfast opposition to World War I brought great scorn down upon him, with people such as Theodore Roosevelt calling him a traitor and others labeling him a German sympathizer. Nonetheless, he survived the war and the Red Scare, standing up for working people throughout the period, including vociferously opposing the imprisonment of socialist leader Eugene Debs for speaking out against the war.
A 1982 historians poll of the greatest senators ever ranked LaFollette tied for first with Henry Clay. That's a remarkable achievement given the people who have served in that body.