If a senator dies, resigns, or is expelled from office, the manner in which the vacancy is filled depends on state law, the U.S. Constitution, and the Senates rules and precedents. Here are some possible scenarios:

• Governor appointment: In 46 states, the governor has the power to appoint a temporary replacement senator. In most of those states, the governor can appoint whomever they wish, with the appointee serving until a successor is elected to fill out the rest of the term. However, some states require the governor to appoint a replacement of the same political party as the previous incumbent.

• Special election: Some states require a special election to fill a vacancy. For example, if the next general election is not to be held within six months after the date of the occurrence of the vacancy, the governor shall call a special primary election and a special general election to fill the vacancy.

• State legislature appointment: Prior to 1913, when state legislatures held the power to elect senators, the legislature, if in session, could fill a vacancy by electing a new senator to serve out the rest of the term. If a vacancy occurred while the state legislature was not in session, the executive thereof may make temporary appointments until the next meeting of the Legislature, which shall then fill such vacancies.

The Seventeenth Amendment to the U.S. Constitution, which was ratified in 1913, allows state legislatures to empower the governor to appoint a replacement to complete the term or to hold office until a special election can take place. The amendment also requires that senators be elected by popular vote rather than by state legislatures.

In summary, the process for filling a Senate vacancy varies by state and can involve a governor appointment, a special election, or a state legislature appointment.