PHILADELPHIA — Naked ballots didn’t cost Joe Biden the 2020 election. That doesn’t mean Democrats are in the clear this year. A down-to-the-wire U.S. Senate race in Pennsylvania has renewed some attention — albeit less than two years ago — on how mail ballots can be rejected in the state if voters make mistakes filling them out and sending them back. In addition to “naked ballots” tossed for missing required inner secrecy envelopes, voters’ ballots can be thrown out for missing the deadline, forgetting to sign them, and other errors. The difference between votes cast and votes actually counted...