Each state has two Senators who are elected to serve six-year terms. Every two years one third of the Senate is up for reelection. To be able to run in an election for the Senate one must be 30 years old by the time one takes the oath of office, a citizen of the U.S. for at least nine years, and a resident of the state from which one is elected. These qualifications were established in Article I, Section 3 of the Constitution.

Most states have primary elections to decide which candidates will be on the November general election ballot. Some state parties hold conventions in conjunction with the primary. If a candidate is unopposed, there may not be a primary election. Those who represent a major political party are automatically placed on a state's primary ballot. Minor party candidates are chosen by their party's rules while independent candidates nominate themselves. Independent candidates and those representing minor parties must meet various state requirements to be placed on the general election ballot. An example of this would be to submit a petition with signatures from a certain number of registered voters.

Before 1913 Senators were chosen by their state legislatures. The Founding Fathers believed that since the Senators represented the state, the state legislature should elect them. The 17th Amendment to the Constitution requires Senators to be elected by a direct vote of those she or he will represent. Election winners are decided by the plurality rule. That is, the person who receives the highest number of votes wins. In some states, this may not necessarily be a majority of the votes. For example, in an election with three candidates, one candidate may receive only 38% of the vote, another 32%, and the third 30%. Although no candidate has received a majority of more than 50% of the votes, the candidate with 38% wins because she or has the most votes (the plurality). In other states, if there is no majority, there is a run off election between the top two candidates.