Well before the delegates to the Constitutional Convention had reached "the Great Compromise" that accorded the states equal votes in the Senate, they had already decided much about the upper house. They determined that the state legislatures would choose the Members of the Senate from their respective states; that it would have fewer Members than the lower house; and that the Members of the Senate would serve longer terms. By these mechanisms, the delegates integrated the states into the national legislative process, "protected" and "preserved" the states, provided for a forum to represent "the great mercantile interest," and made the Senate's membership more "permanent," in order to modify the more "transient impressions" that would influence the House. They perceived the Senate to be a more deliberative body. The House of Representatives, the Framers thought, would initiate most legislation, whereas the Senate was to be a corrective and a refinement of what emanated from the House.

Various delegates suggested terms ranging from three to nine years. James Madison argued for a longer term "to give the Govt. that stability which was every where called for." Most delegates seemed to support a term of seven years, but after Alexander Hamilton proposed a complex system of rotation, Hugh Williamson of North Carolina suggested a term of six years "as more convenient for Rotation then 7 years." After some hesitation, the delegates agreed to six-year staggered terms.

The first Senate was able to reach a quorum on April 6, 1789, and immediately counted the electoral ballots that elected George Washington President. On May 13, they divided themselves into three geographically balanced classes, with no two Senators from the same state in the same class. Then, the Senate resolved that "three papers of an equal size, numbered 1, 2, and 3, be, by the Secretary, rolled up and put into a box," and drawn by three Senators representing the previously assigned classes. The class drawing "1" would vacate at the end of two years, "2" at the end of four, and "3" at the end of six. New states' Senators would be allocated among the classes. Thus began the institution of staggered terms by which the Senate continues to be elected, now through the terms of the Seventeenth Amendment.

At the Convention, only James Wilson had objected to granting governors the power to make appointments to the Senate if there were a sudden vacancy and the legislature was not in session. He thought the device contrary to the principle of the separation of powers. Edmund Randolph, however, declared that the provision was "necessary in order to prevent inconvenient chasms in the Senate," and the Convention agreed.

This provision was modified by the Seventeenth Amendment, which now, in the case of a vacancy, requires governors to “issue writs of election to fill such vacancies.”