Senators were initially elected by state legislatures. By the beginning of the 20th century, legislative assemblies of at least 29 states provided the population with the right to elect senators through referenda. Following the adoption of the Seventeenth Amendment to the Constitution in 1913, all senators are elected popularly.
Each senator is elected for a term of six years. The terms are staggered so that a third of the Senate is re-elected every two years. Senators of the 1st Congress were divided into three-thirds (which are called Classes). The term of the senators of the 1st Class was two years, of the 2nd – four, and of the 3rd – six, starting from the 2nd Congress the senators of all three Classes are elected for six years. The terms are distributed in such a way that both seats from the same state are not contested in the same elections, except when a vacancy is filled before the deadline expires.
The US Constitution (1st article, 4th section, paragraph 2) initially set the date of the congress on convocation on the third day of December. The Twentieth Amendment to the Constitution (1933) set this date to January 3rd unless Congress elects another day by law. The Twentieth Amendment also states that Congress should convene at least once a year, setting meeting dates and its own schedule at its discretion. The Constitution provides the President of the United States to convene Congress at his/her discretion in exceptional cases.
U.S. Senate elections are held in even years on the first Tuesday after the first Monday of November and coincide with elections to the House of Representatives. Senators are elected from the state as a whole. Most states (since 1970) initially run primaries from the Republican and Democratic parties. The winner is the candidate with the simplest majority of votes. The National Mail Voter Registration Form can be used to register U.S. citizens to vote. By the way, Montana is the only state that allows late voter registration. Most states require a person to register 30 days prior to an election to be eligible to vote.
According to the Seventeenth Amendment to the US Constitution, a vacancy vacated before the expiration of the term of office is filled in special, extraordinary elections. In many states, the governor has the opportunity to appoint a deputy senator prior to the election. Whenever a vacancy is vacated before the term expires, the Senate Post Secretary sends one of three forms to the Governor of the State to certify the need to appoint or elect a new Senator. If the elections for two meta senators coincide, each seat is disputed separately. A senator, elected midway through the term, assumes office as soon as possible after election and serves until the expiration of the initial six-year term (i.e., not a full term).
The Seventeenth Amendment allows state legislatures to give their governors the power to appoint a senator before special elections. The senator appointed temporarily may participate in special elections for the same position. As of 2015, 45 states have provided their governors with the appointment of interim senators. In 37 of these states, special elections are held every two years on the next single congressional election day. In the remaining nine – rather, beyond the two-year cycle. In four states (Arizona, Hawaii, Utah, and Wyoming), the governor is required to appoint an interim senator from the same party as the outgoing senator. Oregon, Wisconsin, Oklahoma and Massachusetts do not allow the governor to appoint a temporary senator, and the position remains vacant until special elections.