Two United States Senate elections (a regular and a special election) were held in Illinois on March 26, 1913. The two elections were interconnected through a compromise made to elect a Democrat in the regular election and a Republican in the special election.
The elections saw the election of Democrat J. Hamilton Lewis to a full term in the state's class 2 United States Senate seat in a regular election, and Republican Lawrence Y. Sherman to the state's class 3 United States Senate seat in a special election.
On March 26, in a compromise arranged by governor Dunne, the General Assembly elected Democrat J. Hamilton Lewis to fill the full-term seat and Republican Lawrence Y. Sherman to fill the two remaining years of a vacancy that had just recently opened. This broke a deadlock on the matter that had been in place since February 11.
Non-binding preference primaries were held April 9, 1912, which informed the legislature of the preferred candidate of the voters that participated in each party's primaries. They coincided with binding primaries held for other offices.
While the party was eligible to hold a primary, no Socialist primary was held for the office of U.S. Senator.
The Illinois General Assembly eventually elected the Democratic nominee, Congressman J. Hamilton Lewis March 26, 1913, who had previously won the Democratic advisory primary as the sole candidate on the ballot. Before their conclusive March 26 vote, after a compromise was stricken, the Illinois legislature had twelve-times cast deadlocked ballots for the class 2 senate seat.
Illinois legislative vote, class 2 (March 26, 1913)