According to the United States Constitution, each state gets to have two senators that serve in the United States Senate. The number of representatives in the House of Representatives depends on the population of the state. As a result, the most populous states in the country have the same representation in the Senate as the least populated states. With 50 states in the country and two senators for each state, there are 100 senators in the United States Senate.
Senators serve terms of six years; if they wish to continue serving after that time, they must be re-elected by the citizens of their states. However, imagine if every single senator came up for election at the same time – what could happen is that all of the people who serve in the Senate could be brand new to the job, all at the same time! To make sure that this situation does not happen and that there are always experienced senators present, there are elections every two years. One-third of Senate seats go up for election every two years, so there is always some continuity in terms of who is serving in the Senate.
Most senators belong to either the Democratic or Republican parties. If a state is heavily Republican, its citizens will likely elect Republican senators. If a state is heavily Democratic, its citizens will likely elect Democratic senators. Nevertheless, the greatest predictor of whether or not a senator will be re-elected is not if his or her party corresponds with the party that is most favored by the state’s citizens. The greatest predictor of whether or not a senator will be re-elected is if he or she is already a sitting senator.
Why? Because most people do not know what policies their senators have voted on or what their political views are. They may know the senators’ names, and they are likely to vote for them because they recognize those names. Incumbency is the greatest asset a senator can have in order to be re-elected.
Sometimes, senators do change parties after they are already in office. A Democratic senator may become a Republican, and a Republican senator may become a Democratic. These switches can be very controversial among voters and can also impact whether or not the senator is re-elected. Also, some senators are not from the Democratic or Republican parties. They are either Independent (no party affiliation) or belong to a different, smaller party.