55 ILCS 5/1-5014
The number of legal votes cast at any county seat election held under this Division shall be deemed and taken for the purpose of such an election as prima facie evidence of the number of legal voters of that county at that time entitled to vote on the question; but in case it becomes necessary, in consequence of a contest of an election held under this Division, to ascertain the number of voters of the county entitled to vote upon the question, the court in which the contest is pending may ascertain the number of such voters by taking, or causing to be taken, legal evidence, tending to show the actual number of the legal voters of the county entitled to vote upon such question at the time of such election. Circuit courts shall have jurisdiction of all cases of contested election arising under this Division, and may investigate and determine all questions of fraud and fraudulent voting connected therewith, and purge the poll books and returns of all illegal or fraudulent votes; and may investigate and ascertain the total number of legal voters of the county at the time of such election entitled to vote on the question, whether they voted or not, and ascertain and determine whether such election was fairly carried by 3/5 or a majority of all the legal voters of the county, as required by the Constitution, and enter such judgment as the circumstances of the case may require. Any of the legal voters and taxpayers of the county who may desire to do so, as well as the town, city or village to or from which it is proposed to remove such county seat, may be made, or on their petition may become parties to such actions, either as plaintiff or defendant.
55 ILCS 5/1-5014