Legislators resign, take other jobs, get appointed to higher office, or die in office. Our database records an average of about 65 legislative special elections a year over the past decade, more in odd years, and appointments fill still others with no election at all. How a vacant seat gets refilled depends on the state, and the differences shape who holds power between Novembers.
How vacant seats get filled
Two things can happen to a vacant legislative seat: the voters fill it, or someone appoints a replacement. The National Conference of State Legislatures groups the states into two families: special elections, the primary method in 25 states, and appointment for the rest, a category that runs from a governor's pick to a party committee to a vote of the chamber itself. The appointment states are where the variety lives.
Special elections. The seat goes back to the voters, usually within a few months and often with its own compressed primary. These are the off-schedule races our special elections tracker follows, with swing analysis comparing each result to the district's baseline.
Appointment. Someone names a replacement, and who that someone is varies: most often the governor, and in several states a county board of commissioners. In most appointment states the replacement must come from the departing member's party. How long they serve varies too, and this is where a common assumption goes wrong: in some states the appointee serves out the full remaining term, in others only until the next general election, when voters choose someone to finish it, and in a few, like Montana and Utah, it depends on the chamber. "The governor appoints" also overstates the governor's hand. In Hawaii, Idaho, Maryland, North Carolina, Utah, and West Virginia the departing member's party hands the governor a name or a short list, and, so long as the party meets the statute's deadline, the governor's role is to sign it; the real choice belongs to the party.
The party or the chamber picks directly. A few states cut the governor out entirely. Indiana lets a caucus of the vacating member's precinct committee officers choose the replacement, and Ohio's constitution hands the pick to the sitting members of that party in the affected chamber. Colorado is reworking its version: a party vacancy committee has long filled its legislative seats, and a 2025 law adds a new odd-year "vacancy election" for seats that come open between late July of an even year and late July of the next odd year, open only to unaffiliated voters and voters registered with the vacating member's party. Per the bill's fiscal note, the first one can happen in 2027.
Why it matters
The mechanics sound procedural but the stakes aren't. In appointment states, a party can hold a seat for years without ever defending it in a live election, and because most appointment rules require a same-party replacement, the partisan balance usually survives the handoff. In special-election states, every resignation risks a flip. Those off-schedule, low-turnout specials also behave differently from November races, and their swings against the district's baseline are worth watching, which is why our special elections tracker scores every one. The person changes either way, which matters plenty if you work with legislatures.
One structural wrinkle we watch closely: in multi-member districts with designated posts, like Idaho's Seat A and B or Washington's Position 1 and 2, an appointment fills a specific numbered seat, and matching the resigner to the right seat (versus the seat the appointee was assigned to) is one of the more error-prone corners of legislative record-keeping. In at-large multi-member districts there is no seat number at all, and a successor sometimes can't be traced to a single predecessor.
What our tracking shows
The map below reflects observed practice in our own data: which states we've recorded legislative special elections in, and which we've recorded mid-term appointments in, since 2020. Special elections are the primary method in 25 states by statute, but we've recorded special elections in 43, because many appointment states also send some vacancies to the voters under specific conditions, depending on the timing of the vacancy or the chamber. Illinois, for instance, appoints to fill most legislative vacancies but sends a state Senate seat with more than 28 months left to the voters at the next general election. This is practice as tracked, not a statutory classification; statutory details vary by state and chamber, and state law controls in any specific vacancy.
Succession above the statehouse
Vacancies in statewide offices follow different chains entirely. A governorship passes to the Lieutenant Governor in the 45 states that have one. Of the five that don't, Maine and New Hampshire send the governorship to the president of the state senate, and Arizona, Oregon, and Wyoming to the secretary of state. (Arizona voters created a lieutenant governor's office in 2022; its first holder takes office in January 2027.) Other statewide elected offices are usually refilled by the governor, but not always, and the rule can differ office by office within a state. In several states the legislature makes the choice, sometimes only while it is in session, with the governor filling the seat in the interim; in others the governor must appoint the person the vacating officer's own party names, as Utah requires for its statewide offices. Our officeholder timelines record every start reason (elected, appointed, or succeeded), so the provenance of every current member is visible on their seat's page.