Millions of voters in states across the country cast their ballots in the presidential primary on Super Tuesday, leaving little doubt that the November election will be a rematch between President Biden and former President Donald J. Trump. But in a race that is increasingly inevitable, a New York Times/Siena College poll found a critical group of voters who are making the outcome of that race anything but certain. Nate Cohn, The Times’s chief political analyst, explains who these voters are and why they present a particular threat to Mr. Biden.