Republican leaders in Illinois said Monday that the release of federal census data last week proves their claim that the state’s new legislative district boundaries drawn by Democrats violate voting rights laws and have said they would ask a judge to declare the cards invalid.

Democrats did not directly challenge the GOP’s analysis of census data that showed discrepancies between the population estimates the party used to draw the maps and the census data released Thursday. The party said it was reviewing the latest figures and would make changes to the maps if necessary. Any change would require another vote by the Democratic-controlled legislature unless it resulted from a court ruling.

Republicans filed a federal lawsuit in June challenging maps approved by the Democratic-controlled General Assembly and signed by Gov. JB Pritzker in June because they relied on population estimates for the United States. Census Bureau‘s American Community Survey rather than once-a-decade survey results. census counts, which were released on Thursday.

Democrats used the estimated data because of the late release of the detailed census population count. The party faced a June 30 deadline to enact new boundaries or risk a 50-50 chance of ceding control of the mapping process to Republicans.

Because Democrats control the General Assembly as well as the governor’s office, they were able to approve the new borders without any Republican input, just as they did a decade ago.

Maps drawn after the 2010 census contributed to a 73-45 Democratic majority in the House and a 41-18 advantage in the Senate.

The new maps aim to keep Republicans in a superminority in the General Assembly, by holding multiple one-on-one contests where Republican incumbents in the House would be pitted against each other based on the location of their homes.

The GOP said the ACS estimates underestimated the state’s total population by 41,877 people, based on actual census results. Senate Republican Leader Dan McConchie of Hawthorn Woods said the final census figures showed the estimated data used by Democrats was “grossly inaccurate” and “ignored the voting rights of their own constituents.”

House Republican Leader Jim Durkin of Western Springs said the new legislative maps “have been found to be unusable and illegal given the release of U.S. Census data.” He said the mapping process should return to the June 30 deadline and require a bipartisan commission to redraw the maps – a panel that inevitably leads to a coin toss to determine a partisan tiebreaker member.

Republicans said their analysis shows the Democratic map goes well beyond a 10-percentage-point gap allowed by federal law to comply with one-person-one-vote laws. They said the most populous House District was nearly 15% above the district’s average population, while the least populated House District was nearly 15% below the average. That leaves a gap of almost 30%, or three times the maximum, they said.

House and Senate Democrat officials said they are reviewing the new census data to see how it compares to their maps.

In a joint statement, Chicago Democratic Sens. Omar Aquino, chairman of the Senate redistricting committee, and Elgie Sims, vice chairman, said they remain committed to a “fair map that reflects Illinois’ broad racial and geographic diversity.”

“During this review process, if it becomes clear that updates need to be made, we will take appropriate steps to do so,” the senators said in their statement. They called it “a one-time census” conducted during a pandemic and then-President Donald Trump’s failed efforts to keep non-citizens uncounted.

The two also noted the Census Bureau‘s renewed privacy efforts, in which some specific counts that could reveal identity information were blacked out. The policy, called “differential privacy,” can “lead to inaccuracies, especially in more ethnically and racially diverse communities,” they said.

The Mexican American Legal Defense and Education Fund filed a separate lawsuit challenging the maps, also based on the use of estimated data to draw legislative boundaries.

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