The 4-to-3 ruling by the court’s conservative majority will go into effect for Wisconsin’s primary election next month, though its true impact likely won’t be felt until November’s general election. Governor Tony Evers, a Democrat, and Senator Ron Johnson, a Republican, both face close re-election bids. The court adopted a literal interpretation of the state law, finding that returning an absentee ballot to a municipal clerk, as Justice Rebecca G. Bradley wrote for the majority, “does not mean, nor has historically been understood to mean, the delivery of an unattended ballot drop box.” While state law allows absentee ballots to be returned by mail, Judge Bradley wrote, “ballots, however, are not mailboxes.” Municipal officials overseeing Wisconsin elections used the drop boxes for years without controversy before the 2020 election, when about 500 were in place across the state, usually outside public libraries and municipal buildings. After the election, which President Donald J. Trump lost in Wisconsin to Joseph R. Biden Jr. by about 20,000 votes, Mr. Trump’s campaign and supporters filed a series of lawsuits seeking to invalidate the ballots cast because the method of returning ballots was not expressly allowed under state law. In the opinion, Judge Bradley compared Wisconsin’s election to contests rigged by dictators in Syria and North Korea and questioned whether the state’s past elections were legitimate. “Thousands of votes were cast through this illegal method, directly harming the voters of Wisconsin,” he wrote. “The illegality of these drop boxes undermines the people’s faith that the election produced a result that reflects their will. “Wisconsin voters, and all legal voters, are hurt when the institution charged with administering Wisconsin elections fails to follow the law, causing the results to be disputed.” Republicans who control the Wisconsin Legislature are highly unlikely to enact legislation allowing drop boxes. Robin Voss, the powerful speaker of the Wisconsin Assembly, said in September that drop boxes should only be allowed inside the city clerk’s office during regular business hours. “Should we have lockers everywhere that someone could get in without security?” he said in an interview in his office at the State Capitol in Madison. “I don’t think it’s right.” Wisconsin Democrats, who have watched the state Supreme Court and state legislature steadily erode the influence they and Mr. Evers have over the state’s election rules, warned Friday that the state’s most vulnerable voters could not to participate in the democracy of the state. “With today’s decision, the Wisconsin Supreme Court is making it harder to vote. It’s a slap in the face of democracy itself,” said Ben Wikler, chairman of the Wisconsin Democratic Party. “Our freedom to vote is under attack.”