Edward P. Costigan graduated from Harvard University and was admitted to the bar in Salt Lake City. He practiced law in Denver and was elected state representative in 1902. Costigan transitioned from a Republican to a founding member of the Progressive Party in Colorado and was appointed to the United States Tariff Commission. He later served in the U.S. Senate as a Democrat, focusing on honest elections, local option laws, and labor rights.

Edward P. Costigan's political identity is marked by an ideological fluidity that defies conventional party lines, transitioning from Republican to Progressive to Democrat. His legislative record reveals a calculated approach to balancing industrial interests with regulatory reforms, as seen in his unexpected alignment with opposing factions on environmental regulations. Costigan's rhetorical style is characterized by a focus on local governance and grassroots advocacy, which he frames as essential to honest and effective democracy.