Herbert David Croly was an intellectual leader of the progressive movement as an editor, political philosopher and a co-founder of the magazine The New Republic in early twentieth-century America. His political philosophy influenced many leading progressives including Theodore Roosevelt, Adolph Berle, as well as his close friends Judge Learned Hand and Supreme Court Justice Felix Frankfurter.

Progressivism in the United States is a political philosophy and reform movement. Into the 21st century, it advocates policies that are generally considered social democratic and part of the American Left. It has also expressed itself with right-wing politics, such as New Nationalism and progressive conservatism. It reached its height early in the 20th century. Middle/working class and reformist in nature, it arose as a response to the vast changes brought by modernization, such as the growth of large corporations, pollution, and corruption in American politics. Historian Alonzo Hamby describes American progressivism as a "political movement that addresses ideas, impulses, and issues stemming from modernization of American society. Emerging at the end of the nineteenth century, it established much of the tone of American politics throughout the first half of the century."