There are 55 Republicans and 45 Democrats serving six-year terms in the U.S. Senate. The terms are staggered so that one-third of the seats are up for election every two years.
All members serve two-year terms.
State laws vary as to whether terms of office are two or four years, and as to whether incumbents are eligible to serve a second or third term. State laws also determine the rules for state and local level races, for example, mayor or school board member, many of which are run as nonpartisan, meaning candidates do not align themselves with any political party. All 50 states and the District of Columbia set their own dates this year for primary elections, which allow the Democratic and Republican parties to select their candidates for the November 3 general election. The primary calender runs from March 10 to October 3. Some states allow a party nominee for any position to be selected in a one-time plurality vote, while others, which require a majority of votes cast for selection, hold runoff elections.