Nine months ago, when followers of President Trump stormed the Capitol to halt the election of President Biden, the insurrection appeared to be the work of an extremist fringe led by right-wing militias and pro-Trump zealots. But election denialism, the movement that spawned the uprising, has turned out to be much larger, more durable and every bit as worrisome as the violence of Jan. 6. Stoked relentlessly by Trump, the belief that Biden stole the election has become a tenet of faith for most Republican voters. In fact, Biden won the election decisively — whether measured by the popular vote...