With the 1913 ratification of the Seventeenth Amendment, which provided for popular election of the Senate, the states acquired the option of filling Senate vacancies either by election or by temporary gubernatorial appointment:
When vacancies happen in the representation of any State in the Senate, the executive authority of such State shall issue writs of election to fill such vacancies: Provided, That the legislature of any State may empower the executive thereof to make temporary appointments until the people fill the vacancies by election as the legislature may direct. Aside from the death or resignation of individual Senators, vacancies may also occur when a newly elected administration is inaugurated. In 2008-2009, for instance, four vacancies were created following the presidential election: two in connection with the election of Senators Barack H. Obama and Joseph R. Biden as President and Vice President, and two more when Senator Hillary Rodham Clinton was nominated to be Secretary of State and Senator Kenneth L. Salazar of Colorado was nominated to be Secretary of the Interior. On January 20, 2017, the President nominated Alabama Senator Jeff Sessions for the office of Attorney General of the United States. Senator Sessions was confirmed by the Senate on February 8, 2017; he resigned from the Senate the same day and was sworn in the following day, February 9. Senator Jones will serve for the balance of the term, which expires on January 3, 2021. Minnesota

On December 6, 2017, Minnesota Senator Al Franken announced his intention to resign from the Senate. In accordance with Minnesota law, Governor Mark Dayton announced on December 12 that he would appoint Lieutenant Governor Tina Smith to fill the vacancy until a special election could be held. Senator Franken resigned on January 2, 2018, and Senator Smith was sworn in the next day, January 3. On March 21, Governor Phil Bryant announced his appointment of Mississippi Agriculture Secretary Cindy Hyde-Smith to fill the vacancy until a special election. Senator Hyde-Smith, who was sworn in on April 9, 2018, brings the number of women Senators to a historical record of 23. Special elections for Mississippi Senate seats have certain distinctive characteristics: they are nonpartisan; there is no primary—all qualified candidates contest the special general election; and, a majority of votes is required to win. If no candidate wins a majority, the two who gained the most votes would contest a runoff election held three weeks later, on November 27. The most widespread practice is for governors to appoint temporary Senators who hold the seat until the next statewide general election, at which time a special election is held to fill the seat for the balance of the term. The National Conference of State Legislatures identifies two variations within this larger category: 
36 states that provide for gubernatorial appointments to fill Senate vacancies, with the appointed Senator serving the balance of the term or until the next statewide general election; and 9 states that provide for gubernatorial appointments, but also require a special election on an accelerated schedule, often within a relatively short period after the vacancy occurs. Filling Vacancies by Appointment through the Next General Election

The 36 states listed below authorize their governors to fill Senate vacancies by appointment, with the temporary Senator serving the balance of the term or until a special election is held concurrently with the next statewide general election. This provision is intended to reduce the length of time an appointed Senator holds office before being replaced by an elected successor. Alabama authorizes the governor to fill a Senate vacancy by appointment. Mississippi authorizes the governor to fill a Senate vacancy by appointment until a special election has been held; if less than one year remains on the prior incumbent's term, the appointee serves the balance of the term. At the time of this writing, April 9, 2018, the Senate's records currently identify 198 appointments to the office of U.S. Proposals to Require that Senate Vacancies Be Filled by Election Only—2009-2011

Following controversies that arose in connection with appointments to fill Senate vacancies in 2008 and 2009, particularly with respect to the Illinois Senate vacancy created by the election of Senator Barack H. Obama as President, proposals to eliminate or curtail gubernatorial power to fill Senate vacancies by appointment were introduced in the 111 th Congress and in a number of state legislatures. 899 would have provided that
when the President of the U.S. Senate issued a certification that a vacancy existed in the Senate, a special election to fill the vacancy would be held not later than 90 days after the certification was issued; the election would be conducted in accordance with existing state laws; and a special election would not be held if the vacancy were certified within 90 days of the regularly scheduled election for the Senate seat in question, or during the period between the regularly scheduled election and the first day of the first session of the next Congress. In addition, as noted earlier in this report, North Carolina in 2013, and Maryland in 2016, enacted legislation that required appointments to fill Senate vacancies be from the same political party as the previous incumbent.
United States Senators serve a term of six years. Vacancies occur when an incumbent Senator leaves office prematurely for any reason; they may be caused by death or resignation of the incumbent, by expulsion or declination (refusal to serve), or by refusal of the Senate to seat a Senator-elect or -designate.
Aside from the death or resignation of individual Senators, Senate vacancies often occur in connection with a change in presidential administrations, if an incumbent Senator is elected to executive office, or if a newly elected or reelected President nominates an incumbent Senator or Senators to serve in some executive branch position. The election of 2008 was noteworthy in that it led to four Senate vacancies as two Senators, Barack H. Obama of Illinois and Joseph R. Biden of Delaware, were elected President and Vice President, and two additional Senators, Hillary R. Clinton of New York and Ken Salazar of Colorado, were nominated for the positions of Secretaries of State and the Interior, respectively. Following the election of 2016, one vacancy was created by the nomination of Alabama Senator Jeff Sessions as Attorney General. Since that time, one additional vacancy has occurred and one has been announced, for a total of three since February 8, 2017.
As noted above, Senator Jeff Sessions resigned from the Senate on February 8, 2017, to take office as Attorney General of the United States. The governor of Alabama appointed Luther Strange III to fill the vacancy until a successor was elected. Doug Jones was elected at the December 12, 2017, special election; he was sworn in on January 3, 2018, and will serve through the balance of the term, which expires in 2021.
Senator Al Franken of Minnesota resigned from the Senate on January 2, 2018. On December 12, 2017, Minnesota Lieutenant Governor Tina Smith was appointed by Governor Mark Dayton to fill the vacancy. Senator Smith was sworn in on January 3, 2018. She will serve until a special election is held on November 6, 2018, to fill the seat for the balance of the term, which expires in 2021.
Senator Thad Cochran of Mississippi resigned from the Senate on April 1, 2018. Governor Phil Bryant appointed Cindy Hyde-Smith to fill the vacancy. Senator Hyde-Smith was sworn in on April 9, 2018. She will serve until a nonpartisan special election contested by all qualified candidates is held on November 6. A majority of votes is required to elect. If no candidate wins a majority, the two who gained the most votes will contest a November 27 runoff. The winning candidate will serve for the balance of the term, which expires in 2021. Senator Hyde-Smith brings the number of women Senators to a record total of 23.
The use of temporary appointments to fill Senate vacancies is an original provision of the U.S. Constitution, found in Article I, Section 3, clause 2. The current constitutional authority for temporary appointments to fill Senate vacancies derives from the Seventeenth Amendment, which provides for direct popular election of Senators, replacing election by state legislatures. It specifically directs state governors to "issue writs of election to fill such vacancies: Provided, that the legislature of any state may empower the executive thereof to make temporary appointment until the people fill the vacancies by election as the legislature may direct." Since ratification of the Seventeenth Amendment in 1913, the Senate records currently identify 198 appointments to fill vacancies in the office of U.S. Senator.
During the period since ratification of the Seventeenth Amendment, most states have authorized their governors to fill Senate vacancies by temporary appointments. At present, in 35 states, these appointees serve until the next general election, when a permanent successor is elected to serve the balance of the term, or until the end of the term, whichever comes first. Ten states authorize gubernatorial appointment, but require an ad hoc special election to be called to fill the vacancy, which is usually conducted on an accelerated schedule, to minimize the length of time the seat is vacant. The remaining five states do not authorize their governors to fill a Senate vacancy by appointment. In these states, the vacancy must be filled by a special election, here again, usually conducted on an accelerated schedule. In one notable detail concerning the appointment process, six states require their governors to fill Senate vacancies with an appointee who is of the same political party as the prior incumbent.
Following the emergence of controversies in connection with the Senate vacancy created by the resignation of Senator Barack Obama in 2008, several states eliminated or restricted their governors' authority to fill Senate vacancies by appointment, while both legislation and a constitutional amendment that would have required all Senate vacancies to be filled by special election were introduced in the 111th Congress. None of these measures reached the floor of either chamber, however, and no comparable measures have been introduced since that time.