[1][3] Incumbent Senator Paul Douglas, a Democrat seeking a fourth term, was defeated by Republican Charles H. Percy.

All 24 Illinois seats in the United States House of Representatives were up for election in 1966.

Democrat Adlai Stevenson III was elected to succeed him in office.

[1][4] Third-term incumbent Democrats Kenney E. Williamson and Frances Best Watkins lost reelection.

In order to be approved, legislatively referred state statutes required the support of a majority of those voting on the amendment.

[1] In order to be placed on the ballot, proposed legislatively referred constitutional amendments needed to be approved by two-thirds of each house of the Illinois General Assembly.

[1] General Banking Law Amendment was approved by voters as a legislatively referred state statute.