Republican Philander C. Knox was appointed to the United States Senate in June 1904 after the death of Matthew Quay. Knox was subsequently elected to a full term in the Senate by the Pennsylvania General Assembly, consisting of the House of Representatives and the Senate, in January 1905. Knox served in the U.S. Senate until his resignation on March 4, 1909, to become United States Secretary of State in the William Howard Taft administration, leaving the seat vacant until a successor was elected.
Following the resignation of Sen. Philander C. Knox, the Pennsylvania General Assembly convened on March 16, 1909, to elect a new Senator to fill the vacancy. The results of the vote of both houses combined are as follows:
|Republican||George T. Oliver||201||78.21|
| Pennsylvania U.S. Senate election (Class I)