When Fulton County, Ga., poll manager Suzi Voyles sorted through a large stack of mail-in ballots last November, she noticed an alarmingly odd pattern of uniformity in the markings for Joseph R. Biden. One after another, the absentee votes contained perfectly filled-in ovals for Biden — except that each of the darkened bubbles featured an identical white void inside them in the shape of a tiny crescent, indicating they’d been marked with toner ink instead of a pen or pencil. Adding to suspicions, she noticed that all of the ballots were printed on different stock paper than the others she...