"All states, territories, and districts require special elections to fill any vacant House seats during the first session of a Congress. During the second session of a Congress, however, procedures often vary depending on the amount of time between the vacancy and the next general election. Section 8 of Title 2, United States Code, provides that a state governor can cause a special election in extraordinary circumstances; namely, a crisis in which the number of House vacancies exceeds 100." https://history.house.gov/Institution/Vacancies-Successors/Vacancies-Successors/