Iowa constitution
Article II
Section 1

Every citizen of the United States of the age of twenty-one years, who shall have been a resident of this state for such period of time as shall be provided by law and of the county in which he claims his vote for such period of time as shall be provided by law, shall be entitled to vote at all elections which are now or hereafter may be authorized by law. The general assembly may provide by law for different periods of residence in order to vote for various officers or in order to vote in various elections. The required periods of residence shall not exceed six months in this state and sixty days in the county.