Key Takeaways: Vacancies in Congress
Vacancies in the U.S. Congress occur when a senator or representative dies, resigns, retires, is expelled, or is elected to another office before the end of their regular term.
Most vacancies in the Senate can be filled immediately through an appointment made by the governor to the former senator’s state.
Vacancies in the House can take as long as six months to fill, because representatives can only be replaced through a special election.
https://www.thoughtco.com/how-vacancies-in-congress-are-filled-3322322
Vacancies in the House
Vacancies in the House of Representatives typically take far longer to fill. The Constitution requires that member of the House be replaced only by an election held in the congressional district of the former representative.
"When vacancies happen in the Representation from any State, the Executive Authority thereof shall issue Writs of Election to fill such Vacancies." – Article I, Section 2, Clause 4 of the U.S. Constitution
According to the U.S. Constitution and state law, the governor of the state calls for a special election to replace the vacant House seat. The full election cycle must be followed including political party nominating processes, primary elections and a general election, all held in the congressional district involved. The entire process often takes as long as from three to six months.
While a House seat is vacant, the office of the former representative remains open, its staff operating under the supervision of the Clerk of the House of Representatives. The people of the affected congressional district do not have voting representation in the House during the vacancy period. They can, however, continue to contact former representative's interim office for assistance with a limited range of services as listed below by the Clerk of the House.