Illinois constitution
Article III
Section 1

Every United States citizen who has attained the age of
18 or any other voting age required by the United States for
voting in State elections and who has been a permanent
resident of this State for at least 30 days next preceding
any election shall have the right to vote at such election.
The General Assembly by law may establish registration
requirements and require permanent residence in an election
district not to exceed thirty days prior to an election. The
General Assembly by law may establish shorter residence
requirements for voting for President and Vice-President of
the United States.