“The conception of political equality from the Declaration of Independence, to Lincoln’s Gettysburg Address, to the Fifteenth, Seventeenth, and Nineteenth Amendments can mean only one thing — one person, one vote,” Justice William O. Douglas wrote for the court in 1963, referring to the amendments that extended the franchise to blacks and women and required the popular election of the Senate.

The rulings revolutionized American politics — everywhere but in the Senate, which the Constitution protected from change and where the disparities in voting power have instead become more extreme.

A Barrier to Change

In his memoirs, Chief Justice Earl Warren described the cases from the 1960s establishing the equality of each citizen’s vote as the most important achievement of the court he led for 16 years. That made them more important in his view than Brown v. Board of Education, which ordered the desegregation of public schools, and Gideon v. Wainwright, which guaranteed lawyers for poor people accused of serious crimes.

“Legislators represent people, not trees or acres,” Chief Justice Warren wrote for the court in 1964, rejecting the argument that state senators, like federal ones, could represent geographic areas with varying populations. “Legislators are elected by voters, not farms or cities or economic interests.”

Applying that principle to the Senate would be very hard. Even an ordinary constitutional amendment would not do the trick, as the framers of the Constitution went out of their way to require states to agree before their power is diminished. Article V of the Constitution sets out the procedure for amendments and requires a two-thirds vote of both houses of Congress or action by two-thirds of state legislatures to get things started. But the article makes an exception for the Senate. “No state, without its consent, shall be deprived of its equal suffrage in the Senate,” the article concludes.

The United States Senate is hardly the only legislature that does not stick strictly to the principle of equal representation. Political scientists use the term “malapportioned” to describe the phenomenon, and it is common around the world.

But the Senate is in contention for the least democratic legislative chamber. In some other countries with federal systems, in which states or provinces have independent political power, a malapportioned upper house may have only a weak or advisory role. In the United States, the Senate is at least equal in power to the House, and it possesses some distinctive responsibilities, like treaty ratification and the approval of presidential appointments. A recent appeals court decision severely limiting the president’s power to make recess appointments, if it stands, will further increase the Senate’s power.