The classes of U.S. senators are the three groups the Senate’s 100 seats are divided into, each on its own six-year election cycle. Class I, Class II, and Class III are staggered so that one class faces voters every two years, and the two senators from any single state always sit in different classes. That structure means about a third of the Senate is on the ballot in any given federal election, and no state ever has both of its Senate seats up in the same year through the regular schedule.
What Each Class Is and When It’s Up
Each class holds either 33 or 34 seats, keeping the three groups nearly equal as the Constitution requires. The current split is 33-33-34.
- Class I (33 seats). Last elected in November 2024. Terms run through January 3, 2031. Next regular election: 2030.1United States Senate. Class I – Senators Whose Term of Service Expire in 2031
- Class II (33 seats). Last elected in November 2020. Terms expire January 3, 2027. Next regular election: November 2026.2United States Senate. Class II – Senators Whose Terms of Service Expire in 2027
- Class III (34 seats). Last elected in November 2022. Terms run through January 3, 2029. Next regular election: 2028.3United States Senate. Class III – Senators Whose Terms of Service Expire in 2029
The pattern repeats every six years. Voters always see roughly a third of the Senate contested alongside the entire House.4USAGov. Congressional Elections and Midterm Elections
Which States Vote in 2026
The 33 Class II seats on the ballot in November 2026 belong to Alabama, Alaska, Arkansas, Colorado, Delaware, Georgia, Idaho, Illinois, Iowa, Kansas, Kentucky, Louisiana, Maine, Massachusetts, Michigan, Minnesota, Mississippi, Montana, Nebraska, New Hampshire, New Jersey, New Mexico, North Carolina, Oklahoma, Oregon, Rhode Island, South Carolina, South Dakota, Tennessee, Texas, Virginia, West Virginia, and Wyoming.2United States Senate. Class II – Senators Whose Terms of Service Expire in 2027 Any state not on that list won’t hold a regular Senate election in 2026 unless a vacancy triggers a special election.
Why a State’s Two Senators Sit in Different Classes
The Senate has followed one strict rule since 1789: a state’s two senators must belong to different classes. The original committee that sorted the first senators allocated them “so that both senators from the same state should not be in the same class, so that there never should be a vacancy, at the same time, of the seats of both senators.”5Legal Information Institute. U.S. Constitution Annotated – Article I, Section 3, Clause 2 – Staggered Senate Elections The result: a state’s two Senate seats come up in different years, so no single wave election can replace both at once.
The same logic governs new states. When Hawaii joined the Union in 1959, its two incoming senators drew from a wooden box holding the numbers “one and three,” landing them in Class I and Class III. That draw produced the current 33-33-34 balance across the three classes.6United States Senate. Senators Receive Class Assignments
How Vacancies Fit Into the Schedule
When a senator leaves office early through resignation, death, or expulsion, the replacement does not start a fresh six-year term. The successor serves only the remainder of the original term attached to that seat’s class.7United States Senate. Filling Vacancies A senator who departs four years in leaves two years on the clock; whoever fills the seat serves those two years, and the seat then returns to the ballot at the next scheduled election for its class. The rotation stays synchronized regardless of turnover.
What differs is how states handle the gap. Most allow the governor to appoint a temporary replacement. Four states (Kentucky, North Dakota, Rhode Island, and Wisconsin) prohibit gubernatorial appointments entirely, leaving the seat empty until voters fill it. Ten states require the governor to appoint someone from the same political party as the departing senator. In Kansas and Utah, the governor picks from a shortlist submitted by the state legislature. The 17th Amendment, ratified in 1913, established this framework by allowing state legislatures to authorize gubernatorial appointments during vacancies.8Constitution Annotated. Amendment XVII
Where the Class System Comes From
Article I, Section 3 of the Constitution required the very first Senate to divide its members into three groups “as equally as may be,” with the first group’s seats expiring after two years, the second after four, and the third after six, “so that one third may be chosen every second Year.”9Constitution Annotated. Article I Section 3 Once those staggered initial terms expired, every subsequent term ran the full six years. The framers wanted a chamber insulated from the rapid shifts that could sweep the House in a single election. The rotating class schedule was the mechanism.
The 17th Amendment, ratified April 8, 1913, moved the choice of senators from state legislatures to the voters themselves but left the six-year terms and three-class rotation completely intact.10National Archives. 17th Amendment to the U.S. Constitution – Direct Election of U.S. Senators
What the Staggered Classes Produce
Because only one class faces voters at a time, at least two-thirds of the Senate always carries over from the previous Congress. This is why the Senate is called a “continuous body.” Pending legislation, treaty negotiations, and confirmations don’t reset with each election, and a new senator joins a chamber whose rules, norms, and relationships are already set by colleagues who weren’t on the ballot.
Seniority and Committee Chairs
The staggered system feeds directly into the Senate’s seniority hierarchy. Seniority is measured by length of continuous service, and since the 1840s it has been the dominant method for distributing committee chairmanships and leadership roles. Because the class rotation guarantees most senators survive any given cycle, long-serving members accumulate procedural influence over time. Party conferences use seniority to arrange committee rosters, though internal rules can cap it. Senate Republicans, for example, adopted six-year term limits for committee chairs in 1997.11United States Senate. Seniority
President Pro Tempore
The most visible product of that seniority is the president pro tempore, who presides over the Senate when the vice president is absent and stands third in the presidential line of succession. Since 1890, the position has customarily gone to the longest-serving senator in the majority party. The full Senate elects the president pro tempore, but the seniority convention holds so reliably that the vote is often taken without a recorded tally.