The Illinois Senate is the upper chamber of the Illinois General Assembly, the legislative branch of the government of the State of Illinois in the United States. The body was created by the first state constitution adopted in 1818. Under the Illinois Constitution of 1970, the Illinois Senate is made up of 59 senators elected from individual legislative districts determined by population and redistricted every 10 years; based on the 2020 U.S. census each senator represents approximately 213,347 people. Senate districts are divided into three groups, each group having a two-year term at a different part of the decade between censuses, with the rest of the decade being taken up by two four-year terms. For example, group one elects senators for terms of four years, four years and two years, group two elects senators for terms of four years, two years and four years, and group three elects senators for terms of two years, four years and four years. This ensures that the Senate reflects changes made when the General Assembly redistricts itself after each census.
Usually, depending on the election year, roughly one-third or two-thirds of Senate seats are contested. On rare occasions (usually after a census), all Senate seats are up for election. In contrast, the Illinois House of Representatives is made up of 118 members with its entire membership elected to two-year terms. House districts are formed by dividing each Senate district in half, with each senator having two "associated" representatives.
