[2][1] At his death in 2006, The Guardian called him "the leading theorist of democracy and American exceptionalism";[2] The New York Times said he was "a pre-eminent sociologist, political scientist and incisive theorist of American uniqueness";[1] and The Washington Post said he was "one of the most influential social scientists of the past half century.

"I was in that atmosphere where there was a lot of political talk," Lipset recalled, "but you never heard of Democrats or Republicans; the question was communists, socialists, Trotskyists, or anarchists.

"[11] Lipset's modernization theory has continued to be a significant factor in academic discussions and research relating to democratic transitions.

[16] The Lipset hypothesis has been challenged by Guillermo O'Donnell, Adam Przeworski and Daron Acemoglu and James A. Robinson.

Larry Diamond and Gary Marks argue that "Lipset's assertion of a direct relationship between economic development and democracy has been subjected to extensive empirical examination, both quantitative and qualitative, in the past 30 years.

And the evidence shows, with striking clarity and consistency, a strong causal relationship between economic development and democracy."

Political Man was published and republished in several editions, sold more than 400,000 copies and was translated into 20 languages, including: Vietnamese, Bengali, and Serbo-Croatian.

He argued that the reason for this divergence is that the initial patterns of colonization, the subsequent process of economic incorporation of the new colonies, and the wars of independence varied.

[21] Lipset left the Socialist Party in 1960 and later described himself as a centrist, deeply influenced by Alexis de Tocqueville, George Washington, Aristotle, and Max Weber.

[23][1][24] Lipset was vice-chair of the board of directors of the United States Institute of Peace,[25] a board member of the Albert Shanker Institute, a member of the US Board of Foreign Scholarships, co-chair of the Committee for Labor Law Reform, co-chair of the Committee for an Effective UNESCO, and consultant to the National Endowment for the Humanities, the National Humanities Institute, the National Endowment for Democracy, and the American Jewish Committee.

Lipset was a strong supporter of the state of Israel, and was President of the American Professors for Peace in the Middle East, chair of the National B'nai B'rith Hillel Commission and the Faculty Advisory Cabinet of the United Jewish Appeal, and co-chair of the executive committee of the International Center for Peace in the Middle East.

He worked for years on seeking solution for the Israeli–Palestinian conflict[25] as part of his larger project of research on the factors that allow societies to sustain stable and peaceful democracies.