The mistake put more than 500 people into the system even though they had indicated they were not U.S. citizens, and therefore not eligible to vote.
The Secretary of State's Office said the problem has been fixed.
According to the Illinois State Board of Elections, a total of 545 people were improperly registered to vote by local authorities, and already 161 have been removed from the voter rolls. But that's not enough to quell the criticism.
It was a problem that started in 2018. People visiting Secretary of State facilities checked a box saying they were not U.S. citizens, but still had their information processed through the state's automatic voter registration program.
A computer glitch sent 574 names to the State Board of Elections with 545 people actually getting registered to vote in error.
"Today we need to start the discussion about suspending the automatic voter registration program until we know for certain that this is being implemented correctly," said State Rep. Avery Bourne, (R) Litchfield.
Republicans are calling for an investigation and so is the governor, who also supports holding legislative hearings on what caused the problem.
"I think that it's appropriate again for transparency, for getting all the information, so we know what went wrong. Look I don't think we need to be pointing fingers at anybody about blame, I think it's just look, we have to get it right," said Governor JB Pritzker.
The Secretary of State's Office has owned up to the mistake, but said the glitch has been corrected.
"While we do want to apologize to anyone who was affected by this, the fact of the matter is this is a very small percentage of people who have been registered through automatic voter registration. Less than one tenth of one percent," said Dave Drucker, a spokesman for the Illinois Secretary of State's Office.
More than 600,000 people have taken part in the automatic voter registration program since it became law in 2017. All of the 545 people improperly registered have been notified since the problem was discovered last month, but the State Board of Elections said only 16 actually voted.
"We know who the sixteen individuals are, and we have been able to, so far we've been able to determine that we know of four that had voting histories that predate this," said Matt Dietrich, a spokesman for the State Board of Elections.
He noted that people may have inadvertently checked the wrong box in the process of registering to vote, when in fact they were eligible.
The calls for investigations from the governor on down are about ensuring the integrity of the election process and making sure there are no more problems in the future.