Filling U.S. Senate Vacancies: Perspectives and Contemporary Developments [February 13, 2013] [open pdf - 290KB]
"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. This report provides information on current vacancies in the Senate, the constitutional origins of the Senate vacancy clause, the appointment process by which most vacancies are filled, and related contemporary issues. It will be revised and updated to reflect current developments in vacancies, appointments, and special elections."
CRS Report for Congress, R40421