The Wisconsin spring elections were less than a week away, and with the state’s coronavirus death toll mounting, Democrats were challenging Republican plans to hold the vote as scheduled. In an emergency hearing, held via videoconference, John Devaney, a lawyer for the Democrats, proposed a simple compromise: Extend the deadline for mail ballots by six days past Election Day, to April 13, to ensure that more people could vote, and vote safely. “That’s going to be much more enfranchising,” said Mr. Devaney, arguing one of the most politically freighted voting-rights cases since Bush v. Gore from his bedroom in South Carolina as his black lab, Gus, repeatedly interrupted at the door. The presiding federal judge, William M. Conley, agreed, pointing out that clerks were facing severe backlogs and delays as they struggled to meet surging demand for mail-in ballots. Yet with hours to go before Election Day, the Supreme Court reversed that decision along strict ideological lines, a decision based in large part on the majority’s assertion that the Democrats had never asked for the very extension Mr. Devaney requested in court. It was the first major voting-rights decision led by the court’s conservative newest member, Justice Brett M. Kavanaugh, and it was in keeping with a broader Republican approach that puts more weight on protecting against potential fraud — vanishingly rare in American elections — than the right to vote, with limited regard for the added burdens of the pandemic.