25 Pa. Stat. § 2776
Whenever a vacancy shall occur in the office of United States Senator, said vacancy shall be filled for the unexpired term by the vote of the electors of the State at a special election to be held at the time of the next general or municipal election, occurring at least ninety (90) days after the happening of such vacancy, and it shall be the duty of the Governor to issue writs of election to the various county boards of elections and to the Secretary of the Commonwealth within ten (10) days after the happening of said vacancy. Candidates to fill vacancies in the office of United States Senator shall be nominated by political parties, in accordance with the party rules relating to the filling of vacancies, by means of nomination certificates, in the form prescribed in section 630 of this act; and by political bodies, by means of nomination papers in accordance with the provisions of sections 951, 952 and 954 of this act. Said nomination certificates and nomination papers shall be filed in the office of the Secretary of the Commonwealth at least sixty (60) days prior to the date of said special election. Until such time as said vacancy shall be filled by an election as herein provided, the Governor of the Commonwealth may make a temporary appointment to fill said vacancy.
25 P.S. § 2776