Clause 2: Classification of senators; Vacancies.

Immediately after they shall be assembled in Consequence of the first Election, they shall be divided as equally as may be into three Classes. The Seats of the Senators of the first Class shall be vacated at the Expiration of the second Year, of the second Class at the Expiration of the fourth Year, and of the third Class at the Expiration of the sixth Year, so that one third may be chosen every second Year; and if Vacancies happen by Resignation, or otherwise, during the Recess of the Legislature of any State, the Executive thereof may make temporary Appointments until the next Meeting of the Legislature, which shall then fill such Vacancies.

After the first group of senators was elected to the First Congress (1789–1791), the senators were divided into three "classes" as nearly equal in size as possible, as required by this section. This was done in May 1789 by lot. It was also decided that each state's senators would be assigned to two different classes. Those senators grouped in the first class had their term expire after only two years; those senators in the second class had their term expire after only four years, instead of six. After this, all senators from those states have been elected to six-year terms, and as new states have joined the Union, their Senate seats have been assigned to two of the three classes, maintaining each grouping as nearly equal in size as possible. In this way, election is staggered; approximately one-third of the Senate is up for reelection every two years, but the entire body is never up for re-election in the same year (as contrasted with the House, where its entire membership is up for reelection every 2 years).

As originally established, senators were elected by the Legislature of the State they represented in the Senate. If a senator died, resigned, or was expelled, the legislature of the state would appoint a replacement to serve out the remainder of the senator's term. If the state legislature was not in session, its governor could appoint a temporary replacement to serve until the legislature could elect a permanent replacement. This was superseded by the Seventeenth Amendment, which provided for the popular election of senators, instead of their appointment by the state legislature. In a nod to the less populist nature of the Senate, the amendment tracks the vacancy procedures for the House of Representatives in requiring that the governor call a special election to fill the vacancy, but (unlike in the House) it vests in the state legislature the authority to allow the governor to appoint a temporary replacement until the special election is held. Note, however, that under the original Constitution, the governors of the states were expressly allowed by the Constitution to make temporary appointments. The current system, under the Seventeenth Amendment, allows governors to appoint a replacement only if their state legislature has previously decided to allow the governor to do so; otherwise, the seat must remain vacant until the special election is held to fill the seat, as in the case of a vacancy in the House.

Clause 3: Qualifications of senators.

No Person shall be a Senator who shall not have attained to the Age of thirty Years, and been nine Years a Citizen of the United States, and who shall not, when elected, be an Inhabitant of that State for which he shall be chosen.

A senator must be at least 30 years of age, must have been a citizen of the United States for at least nine years before being elected, and must reside in the State they will represent at the time of the election.