Sargent Shriver had considered a possible run, but opted against a campaign after President Lyndon B. Johnson asked him to remain in Washington to assist with Great Society programs.

[3] The Republican primary was also held on April 14. Business executive[5] Charles Percy won the nomination against Illinois state treasurer William Scott, and a field of minor candidates.

Illinois secretary of state Charles F. Carpentier was originally running, but dropped-out in January after suffering a heart attack.

[5] Also originally running was Cook County Republican Party chairman Hayes Robertson,[5][6] who had unsuccessfully challenged William Stratton in the 1960 Republican gubernatorial primary.

Robertson also withdrew in January, throwing his support behind Scott.