According to Illinois ' 1970 constitution , senators 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 . . Under the Illinois Constitution of 1970 , senators 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 .