What Are Senate Classes? Class I, II and III Explained

Senate classes are the three groups that U.S. Senate seats are divided into so that only about a third of the chamber is up for election every two years. Class I has 33 seats, Class II has 33 and Class III has 34. Class II is the class on the ballot on November 3, 2026, along with two special elections for Class III seats, for 35 Senate races in total.

The system dates to the first Senate in 1789, and every seat still carries the class it was given then, or when its state joined the Union.

Where do Senate classes come from? #

The Constitution set six-year Senate terms but wanted the chamber to turn over gradually. Article I, Section 3 says that after the first election, senators “shall be divided as equally as may be into three Classes. The Seats of the Senators of the first Class shall be vacated at the Expiration of the second Year, of the second Class at the Expiration of the fourth Year, and of the third Class at the Expiration of the sixth Year, so that one third may be chosen every second Year.” You can read the clause on the Constitution Annotated site.

In May 1789 the senators then seated split themselves into three groups and drew lots to decide which group would serve two, four or six years. After that first round, every seat followed the six-year rhythm of its class. When a senator leaves, the seat’s class doesn’t change; the next person elected to it inherits the same schedule.

Which Senate class is up in 2026, 2028 and 2030? #

Class II elections always fall in midterm years like 2026. Class III elections line up with presidential years like 2028, and Class I seats will be up again in 2030.

In 2026, Class II includes seats in Georgia, Maine, Michigan, New Hampshire, North Carolina and Texas, among others. The two special elections are for Class III seats in Florida and Ohio, vacated when Marco Rubio became Secretary of State and JD Vance became Vice President. The winners of those specials serve only the rest of the term, to January 3, 2029. Our list of which Senate seats are up in 2026 goes race by race.

Going into the election, Republicans hold 53 seats, Democrats 45, and two independents caucus with Democrats.

Why does the class system matter for who controls the Senate? #

Because each class covers a different set of states, the battlefield changes every two years. One class may force a party to defend many seats in states that lean toward the other side, while the next class hands it mostly safe seats. People call this a good or bad “Senate map,” and it can matter as much as the national mood.

The timing matters too. Class II seats are always decided in midterms, when turnout is lower and the president’s party has historically lost ground. Class III seats ride with presidential races and their larger turnout. Our explainer on why the president’s party loses seats in midterms covers that pattern.

To follow the Class II races and the two specials, Election Tracker groups every Senate race with published polling on its Senate tab, each with a 30-day polling average and the individual polls underneath. You can pin the races you care about to the top of the tab.

How were new states assigned to a class? #

When a state joins the Union it gets two senators, and the Senate has to fit them into the three classes without unbalancing them. The practice has followed three rules:

1. The two senators go into different classes. A state never has both seats up in the same regular election.
2. They fill the smallest classes. The seats go to whichever classes have the fewest members, keeping the three groups as even as possible.
3. Lots break ties. If two classes are equally small, senators draw lots.

Because a new state’s senators usually join classes that are partway through their cycle, one or both of them start with a shortened first term. Alaska and Hawaii went through this in 1959, drawing lots for their classes. After that first alignment, their seats followed the normal six-year cycle.

What happens to the class when a Senate seat becomes vacant? #

A vacancy never changes a seat’s class. If a senator dies, resigns or is expelled, state law decides how the seat is filled in the meantime. Most states let the governor appoint a temporary senator until a special election, and a few require the appointee to be from the same party as the departed senator. Our guide to how congressional vacancies are filled covers the rules.

The special election winner serves only the rest of the original term. That’s why Florida and Ohio voters pick senators in 2026 for terms ending in January 2029, and then vote for those same seats again in 2028 for a full six years.

Occasionally a state has both of its seats on the ballot in one year: one regular, one special. That doesn’t break the class rule, because the two seats are still in different classes.

Frequently asked questions #

Can a state have both of its senators in the same class? #

No. A state’s two seats are always in different classes, so a single regular election never replaces both at once. Both seats can appear on the same ballot only when one of them is filled by a special election.

Which Senate class is up for election in 2026? #

Class II, with 33 seats, plus special elections in Florida and Ohio for two Class III seats. That’s 35 Senate races on November 3, 2026. Class II winners serve through January 3, 2033.

How many senators are in each class? #

Class I has 33, Class II has 33 and Class III has 34, for 100 senators. That’s as even as 100 seats can be split three ways.

Why do some senators serve less than six years? #

Senators who win special elections serve only the remainder of the term they’re filling. Senators appointed to vacancies serve until a special election is held. The first senators from a new state may also get a shortened first term so their seats line up with their assigned classes.

When is the 2026 Senate election? #

The general election is Tuesday, November 3, 2026, and most primaries are already over. Our 2026 midterm election calendar lists the remaining dates, including runoffs.