While many constitutional amendments have added to the rights held by Americans, changed the balance of power between the federal government and states, or altered elections for the President, the structure of Congress in the written Constitution has barely been touched since 1791. The only constitutional amendment to do so in a substantial way is the Seventeenth Amendment, which removed from state legislatures the power to choose U.S. Senators and gave that power directly to voters in each state.
According to James Madison, giving state legislatures the power to choose Senators provided a “double advantage,” both “favoring a select appointment, and of giving to the State governments such an agency in the formation of the federal government as must secure the authority of the former.” The Federalist No. 62. George Mason argued that state legislative selection gave states the power of self-defense against the federal government. Wendell Pierce argued that the contrast between a state legislatively-appointed Senate and a popularly-elected House would increase the types of interests represented in the federal government. By requiring the consent of two different constituencies to any legislation—the people’s representatives in the House and the state legislatures in the Senate—the composition of the Senate was seen as essential to the system of bicameralism, which would require “the concurrence of two distinct bodies in schemes of usurpation or perfidy.”
Whether state legislative appointment was included in the Constitution to protect state governments, though, is a matter of some dispute. Contemporary legal scholar Terry Smith argues that it was merely the result of the intersection of two other goals, the Great Compromise giving states equally-weighted votes in the Senate and a desire to limit popular representation.
Either way, state legislatures were not given other powers that might have allowed them to more directly control Senators, like the power to recall Senators or to instruct them on how to vote. As a result, scholars like William Riker and Larry Kramer have argued that state legislatures exerted little control over Senators at any point, although more recent work by Todd Zywicki has argued that this is overblown and state legislative control did have a substantial effect on the way the Senate operated. (See Todd Zywicki’s individual explainer on the Seventeenth Amendment.)  However, starting in roughly the 1830s and then more dramatically after the Civil War, the vision the Founders had—in which state legislatures would deliberate over the selection of Senators—began to fray. First, politicians seeking Senate seats began campaigning for state legislative candidates in a process known as the “public canvass.” The result was that state legislative races became secondary to Senate races. The most famous instance of this was the race for Senate in Illinois in 1858, in which Abraham Lincoln faced off with Stephen Douglass despite neither being on the ballot. In 1890s, many states started holding direct primaries for Senate, reducing the degree of influence state legislatures had over selection. Some states went further and began using something known as the “Oregon System,” under which state legislative candidates were required to state on the ballot whether the
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