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The U.S. Supreme Court ruled Monday that Illinois and other states can continue to count mail-in ballots cast by Election Day that arrive afterward, rejecting a challenge by President Donald Trump and Republicans to require ballots be delivered to election authorities by the date of the November general election.

In a 5-4 ruling authored by Justice Amy Coney Barrett, a Trump appointee, the court’s majority found that “while federal law dictates when ballots must be cast, state law dictates when they must be received.”

“The defining element of an ‘election’ — the term used in … federal statutes — has always been the electorate’s choice of candidate,” Barrett wrote. She was joined by Chief Justice John Roberts and the court’s three liberal justices in upholding a Mississippi law allowing ballots cast by Election Day but arriving five days afterward to be counted.

“By setting the day for the ‘election,’ (federal) statutes set the day when the electorate must make its choice,” she wrote. “The electorate’s choice is made when voting is complete, not when ballots are received.”

The ruling upholds Illinois law, which allows mail-in ballots postmarked — or personally attested to — by Election Day to be counted up to two weeks afterward. It also appears to render moot a separate federal court challenge to the Illinois law brought by Republican U.S. Rep. Mike Bost of Murphysboro.

Bost’s suit was assisted by the GOP-aligned legal organization Judicial Watch. Its president, Tom Fitton, called the high court’s ruling “astonishing” and said it “further eviscerates the very notion of Election Day and threatens to make a mess for our nation’s already stressed federal elections.”

Roughly 30 states count at least some absentee ballots mailed by Election Day but received afterward, the opinion noted. A ruling to end the practice threatened to upend state election laws heading into the crucial midterm elections on Nov. 3.

The court’s decision is a blow to Trump, who has long sought to curb mail-in balloting in favor of in-person voting on Election Day. He has contended the practice was rife with fraud without offering evidence — even as he himself has voted by mail in recent elections.

In addition to seeking to curb mail-in voting through executive order and trying to enlist the U.S. Postal Service to ensure delivery of mail-in ballots to legally qualified voters, Trump has pressed for congressional passage of the stalled SAVE America Act. The legislation would require proof of citizenship to register to vote, a photo ID to cast a ballot in federal elections and would sharply restrict mail-in voting to people with illnesses, those traveling or active-duty military personnel.

In a post on his Truth Social account, Trump called the Supreme Court’s ruling a “tremendous loss” for “Voter’s Rights” and said it was “more important than ever to pass THE SAVE AMERICA ACT,” though it does not directly address late-arriving ballots.

“There is only one reason to oppose — CHEATING!” Trump wrote of the proposal, contending “there is a powerful Communist Movement taking place in our Country, one more dangerous than World War I, World War II, Pearl Harbor, or September 11th.”

“All Dumocrats, and our five Republican Senate Hold Outs, Lisa Murkowski, Susan Collins, Thom Tillis, Bill Cassidy, and Mitch McConnell must vote to SAVE OUR COUNTRY. There can be no more excuses!” he wrote. Murkowski is from Alaska and Collins is from Maine. Tillis of North Carolina and McConnell of Kentucky are not seeking reelection; Cassidy of Louisiana lost his primary to a Trump-backed challenger.

Last week, Trump canceled a ceremony to sign a bipartisan affordable housing agreement, saying he wanted to see the SAVE Act passed first. But Senate Republicans say the election measure cannot pass the chamber or bypass filibuster rules.

Trump’s push to federalize election law has run up against the U.S. Constitution and court precedent holding that states are responsible for organizing and conducting elections, though Congress can intervene.

Gov. JB Pritzker, an ardent Trump critic and a potential 2028 Democratic presidential contender, said the nation’s highest court ruled against the president “on an asinine attempt to throw out a law that ensures mail ballots get counted, but his assault is not over.”

“Donald Trump is using every weapon in his arsenal to attack our free and fair elections,” Pritzker, who is seeking a third term as governor, said in a statement from his political campaign. “We cannot look away while the most corrupt president in history attempts to rewrite our election laws to serve his own interests.”

Cook County Clerk Monica Gordon, whose office oversees elections in Cook County suburbs, celebrated the ruling and said mail-in balloting is “reliable, secure, and an essential part of our democratic process.”

“This commonsense approach protects voters from delays beyond their control, including mail disruptions and circumstances that can disproportionately affect seniors, people with disabilities, military service members, and citizens living overseas,” she said.

In an acknowledgment of concerns over election integrity fostered by Trump and his continued false claims that the 2020 presidential election was stolen from him, Barrett’s opinion noted “election fraud and its appearance are serious issues” but “they must be addressed through the democratic process.”

“The question today is not whether requiring ballots to be received by election day is a good or bad idea, the question is whether the idea has made its way into the United States Code,” Barrett wrote. “If varied deadlines for ballot receipt … call for a national solution, the American people must choose it through their elected representatives.”

The Mississippi ballot counting law was challenged by the Republican National Committee, with Trump’s Justice Department joining the case. In a dissent joined by the court’s remaining conservative justices, Justice Samuel Alito contended that “if ballots received after election day are added to the set of ballots that dictate the election’s outcome, the electorate’s choice does not occur on election day and the federal election-day statutes are violated.”

“Allowing absentee ballots to pour in over the days and weeks after election day, by which point preliminary election returns are being publicly reported, creates greater opportunity for fraud and risks further undermining the public’s confidence in election integrity,” Alito wrote, contending the majority’s decision “opens Pandora’s box.”

“Even in the absence of partisan rhetoric, drawn-out ballot-counting ‘induces a large, significant decrease’ in Americans’ trust in elections,” Alito wrote. “By allowing States to continue receiving new ballots during these drawn-out processes, today’s decision will only exacerbate voters’ distrust.”

Tribune reporter AD Quig contributed.