Unlike vacancies in the U.S. House of Representatives, which are required by law to be filled by elections, the U.S. Constitution allows states to choose how to fill vacancies in the U.S. Senate, and all states do so through elections. They vary in two ways, however: whether the vacancy is filled at a regularly scheduled election or at a special election; and whether the governor can make an appointment to fill the vacancy during the period before the election occurs.
Thirty-seven states fill Senate vacancies at their next regularly scheduled general election. The remaining 13 require that a special election be called. Only four states prohibit the governor from making an interim appointment, requiring instead that the seat remain vacant until the next election (whether regular or special) is held. In another three states, the governor may make an appointment to fill the vacancy temporarily, but only under strict conditions.