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 re-election 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 re-election 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 This was superseded by the Seventeenth Amendment, which provided for the popular election of senators, instead of their appointment by the state legislature.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 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. The Supreme Court has interpreted the Qualifications Clause as an exclusive list of qualifications that cannot be supplemented by a House of Congress exercising its Section 5 authority to "Judge... the... Qualifications of its own Members," or by a state