Per the original text of the U.S. Constitution, each state was allotted two U.S. senators selected by the state legislature for staggered six-year terms. After the election of the founding members of the U.S. Senate in 1788, the Senate was divided into three groups, or "classes" (Class 1, Class 2, and Class 3) to stagger the six-year terms of its members. Per Article I, Section 3, Clause 2 of the Constitution, the founding members of Class 1 would serve two years, Class 2 four years, and Class 3 six years. All senators elected thereafter would serve full six-year terms such that one-third of the Senate would be up for re-election every two years.[1] Pennsylvania was assigned a Class 1 seat and a Class 3 seat.[2][3] Since the passage of the 17th Amendment to the Constitution in 1913, U.S. senators are elected directly to six-year terms by the voters of each state at the general election held on Election Day. Special elections may be held to fill mid-term vacancies by electing an individual to serve the remainder of the unexpired term.[4] The next Class 1 U.S. Senate election in Pennsylvania is scheduled for November 5, 2030, and the next Class 3 election in Pennsylvania is scheduled for November 7, 2028.
The list below contains election returns from all twenty Class 1 and twenty-one Class 3 post-17th Amendment U.S. Senate elections in Pennsylvania, including special elections, sorted by year and beginning with the first in 1914 and the most recent in 2024. Incumbent senators are listed as well as elected senators and runner(s)-up in each election, including major third-party candidates (garnering 5% or more of the popular vote). Parties are color-coded to the left of a Senator's or candidate's name according to the key below. The popular vote and percentage margins listed in the "Margin" column are the differences between the total votes received and percentage of the popular vote received by the top two finishers in the corresponding election (i.e. the margin-of-victory of an elected Senator over the nearest competitor).
^In 1922, a special election and a regularly scheduled general election were held simultaneously for the Class 1 seat. David Reed had been appointed in August 1922 to fill the vacancy created by the death of William E. Crow (who had been appointed to fill a vacancy created by the death of Philander C. Knox) until the election of a successor. Reed was subsequently elected both to complete the term ending on March 4, 1923 (upper row), and to a full six-year term beginning on March 4, 1923 (lower row).
^John Heinz died in a mid-air collision between two aircraft, one of which he was aboard, in April 1991. Democrat Harris Wofford was appointed by Governor Robert P. Casey in May 1991 to fill the vacancy. He was subsequently elected to serve the remainder of Heinz's term in the special election of November 1991.
^In 1922, in addition to a special/general election for the Class 1 seat, a special election for the Class 3 seat was held. George W. Pepper was appointed by Governor William Sproul to fill the vacancy following the death of Boies Penrose in December 1921. Pepper was subsequently elected in 1922 to serve the remainder of Penrose's term.
^Although William S. Vare won the 1926 election, his victory was controversial and overshadowed by scandal. As a result, Governor Gifford Pinchot refused to certify the results and Vare (having never been sworn-in) was formally unseated by the U.S. Senate in December 1929. Joseph R. Grundy was appointed later that month by Governor John S. Fisher to fill the vacancy until the next election.
^Specter changed party registration from Republican to Democratic in April 2009. He subsequently lost the primary election to Sestak.
