Aging conservative justices now face the prospect that Republicans won’t control the confirmation process for many yearsWASHINGTON—President Trump put three justices on the Supreme Court during his first term. On Tuesday, voters all but guaranteed that he won’t get a fourth.Democrats’ midterm blowout gave them a Senate majority—with a several-seat cushion—that will make it easy to block Trump judicial nominees, especially for any Supreme Court vacancies that might arise in 2027 or 2028. That prospect casts new uncertainty over the long-term sustainability of the high court’s 6-3 conservative majority, as well as the legacies of the two oldest and most...