U.S. Senate Vacancies: Contemporary Developments and Perspectives [November 30, 2017] [open pdf - 684KB]
"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 appointed Secretaries of State and the Interior, respectively. [...] 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."
CRS Report for Congress, R44781