625 ILCS 5/3-610
Upon receiving an application for a certificate of registration for a motor vehicle from a member of the Congress of the United States from Illinois, accompanied with payments of the registration fees and taxes required under this Act, the Secretary of State instead of issuing to such member number plates as hereinabove provided, shall, if such member so requests, issue to him two number plates as described in this Section. Two duplicate sets of these number plates may be issued if requested and may be used on 2 different motor vehicles. There shall appear, in addition to the designation of the State and the year for which such license was issued, if he is a member of the House of Representatives, the number of the congressional district of such member in the center of the plate followed in the next line by the words "U. S. Congressperson "; if he is the senior Senator from Illinois, the number 1 shall be in the center of the plate followed in the next line by the word "Senator"; and if he is the junior Senator, the number 2 shall be in the center of the plate followed in the next line by the word "Senator".
Such plates may be issued for a 2 year period beginning January 1st of each odd-numbered year and ending December 31st of the subsequent even-numbered years.
625 ILCS 5/3-610