After the Civil War, Senate activity shifted from individuals to groups of members formed into committees. This gave tremendous power to important committee chairmen. By 1900, four senators known as the "Senate Four" dominated the most important Senate committees: Nelson Aldrich of Rhode Island (Finance); William Allison of Iowa (Appropriations); John C. Spooner of Wisconsin (Rules); and Orville Platt of Connecticut (Judiciary).

These close friends met regularly to share information and plan strategy. As one newspaper reporter wrote, "These four men can block and defeat anything that the president or the House may desire." Their links to special interests, and their resistance to policies favored by President Theodore Roosevelt, provoked public concerns that led to calls for reform, including a constitutional amendment for direct election of senators.

“These four men can block and defeat anything that the president or the House may desire.”
— A newspaper reporter of the time, February 19, 1903