Senate classes, and why only one-third of the seats are up

The Constitution requires the Senate to be divided into three classes so that the whole chamber is never on the ballot at once. Senators serve six years. Every two years, one class — about a third of the seats — faces the voters. The Senate’s own terms-of-service page states that rule in plain language. The point is continuity: two classes remain in office while one class is elected.

Class 2 is the class on the regular 2026 ballot. Those senators were last elected in 2020, and their terms expire in January 2027. Class 1 was on the 2024 ballot. Class 3 comes up in 2028. A state hub on this site says whether that state has a Class 2 seat in 2026. Several states do not. Their voters still elect a House delegation, and many elect governors or other statewide offices, but they are not choosing a senator for a new six-year term.

An unexpired term is a different contest. If a senator leaves early, state law decides whether a temporary appointee serves and when voters fill the remainder. The remainder might end in 2027, 2029, or another January, depending on which class the seat belongs to. Ohio’s official ballots for November 3, 2026 include a Senate contest for an unexpired term ending January 3, 2029. That date is the clue that the seat is not the ordinary Class 2 term. Read the term date on the state’s candidate list before treating two Senate lines as the same office.

Winning a Class 2 seat in November 2026 does not by itself decide which party organizes the Senate. Seats in the other two classes stay put. A party that already holds most of those seats can lose a Class 2 race and remain in the majority, or win one and still be short. Ratings of single races, including any Cook Political Report label this site cites on a guide, describe that race only. Adding the labels into a national total would be a model. This site does not build one.

The class system also explains why Senate turnout patterns and House turnout patterns are not interchangeable. A voter in a state with no Senate race still has a House ballot. A voter in a state with a Senate race is choosing a six-year office and, separately, a two-year House seat. Past margins for the last Class 2 election are the 2020 canvass in that state, published by the state and compiled for federal offices by the Federal Election Commission. A 2020 margin is a record of that canvass. It is not a prediction for 2026, and this library does not paste a percentage it has not taken from the canvass it cites.