The formulation of the Senate was the result of the famous Connecticut Compromise (sometimes called the “Great Compromise”) at the Constitutional Convention, which provided for proportional representation of the states in the House and equal representation of the states in the Senate. Each state was to have two Senators, who would be elected by its state legislature, serve for staggered six-year terms, and vote individually. By these devices, the Framers intended to protect the interests of the states as states.

Equal representation of all states in the Senate ensured that the ability of the smaller states to protect their interests would not be seriously impaired. Combined with the bicameral system created by the Constitution, it required that all legislation would have to be ratified by two independent power sources: Representatives of the people in the House and Representatives of the states (regardless of their respective size) in the Senate.

The mode of election impelled Senators to preserve the original federal design and to protect the interests not only of their own states, but, concomitantly, of the states as political and legal entities within the federal system. As Alexander Hamilton declared during the New York ratifying convention in 1788,

When you take a view of all the circumstances which have been recited, you will certainly see that the senators will constantly look up to the state governments with an eye of dependence and affection. If they are ambitious to continue in office they will make every prudent arrangement for this purpose, and, whatever may be their private sentiments of politics, they will be convinced that the surest means of obtaining a re-election will be an uniform attachment to the interests of their several states.

On first blush, per capita voting seems, as Luther Martin argued in the Constitutional Convention, to depart “from the idea of the States being represented in the second branch.” However, the Framers knew from their experiences with block voting under the Articles of Confederation that states had often gone unrepresented because of an evenly divided delegation. They also appreciated that per capita voting could often represent a state’s interests better than block voting, even if occasionally that state’s Senators split their vote. Because their six-year terms of office were to be staggered, and because they were elected by state legislatures which, as James Madison observed in The Federalist No. 63, were continuously “regenerate[d]” by “the periodical change of members,” a state’s two Senators would end up representing somewhat different political moods and sentiments. Elected by shifting majorities in the state legislature, the two Senators, voting per capita, would be able to reflect more accurately the shifting political sentiments of the people in their home states than if they were required to vote as a block.

The Seventeenth Amendment profoundly altered Article I, Section 3 by providing for direct election of the Senate. Several interrelated factors explain its ratification: (1) legislative deadlocks over the election of Senators brought about when one party controlled the state assembly or house and another the state senate; (2) scandals brought on by charges of bribery and corruption in the election of Senators; (3) the growing strength of the Populist movement, with its deep-seated suspicion of wealth and influence and its penchant for describing the Senate as “an unrepresentative, unresponsive ‘millionaires club,’ high on partisanship but low in integrity”; (4) deadlocks in state legislatures that often left states with no elected Senate representation for years; and (5) the rise of Progressivism and its conviction that the solution to the problems of democracy was more democracy—in this case, popular election of Senators.