Federal prosecutors have enormous discretion over which cases to bring. It’s not clear why a fight among county politicians would have interested them, absent the larger shift in the Black Belt’s racial dynamics. The night before the primary, Sessions stationed an F.B.I. agent outside the Perry County post office. The agent saw Albert and Evelyn Turner and a third activist, Spencer Hogue, mailing hundreds of absentee ballots. The F.B.I. opened the ballots and found 75 with candidate votes they suspected had been erased or re-marked.

The government later lowered the number of disputed ballots to 27, but they still sparked a giant investigation. Ten F.B.I. agents fanned out, interviewing more than 1,000 residents about whether they could read or write, whether anyone helped them vote and whether they had altered their own ballots. The Black Belt includes 10 or so counties; the F.B.I. concentrated on the five in which black voters were making strides toward political ascendance. And in each of the five counties, the government targeted longtime black activists and political leaders — figures like Turner.

In October, as part of the investigation and before the general election, Sessions’s office convened a federal grand jury more than 160 miles south, in Mobile, where the jury pool included more white people. Surrounded by F.B.I. agents and police with guns, about 20 black voters from Perry County, many older and some frail, were taken by bus to Mobile, where they were fingerprinted, photographed and questioned by the grand jury about their votes. “This was the most degrading thing,” the Rev. O.C. Dobynes, who accompanied the voters on the bus, told Congress a year later. “To me, it was just simply saying, We are going to scare you into saying what we want you to say.” Asked by a grand juror whether she’d voted absentee for the first time in the September primary, a Perry County resident named Fannie Mae Williams answered: “Uh-huh. First and last.” Two other women told the grand jury they were done with voting, according to the book “Lift Every Voice,” by Lani Guinier, a professor at Harvard Law School.

Albert and Evelyn Turner and Spencer Hogue were indicted in January 1985 on 29 counts, for mail fraud, conspiracy to commit voting fraud and voting more than once. Sessions and his assistants claimed the defendants filled out the disputed absentee ballots themselves. Each faced decades in prison. “It was hell,” remembers Evelyn Turner, now 80. (Her husband died in 2000.) “We had always helped people with voting, for ages, and they trusted us. Why would you mess with someone’s ballot, if you knew it wasn’t what they wanted? We weren’t fools.”

Hank Sanders organized a team that included defense lawyers from the NAACP Legal Defense and Educational Fund and the Southern Poverty Law Center, including Guinier. The Perry case attracted national attention in part because aspects of the prosecution appeared unprecedented. Sessions was the first federal prosecutor to pursue allegations of absentee-voter fraud in a strictly local election, James Liebman, one of the NAACP lawyers, testified to the Senate in 1986, and the first to bring the might of the federal government to bear over a relatively small number of ballots. Liebman also described evidence of absentee-voting irregularities, including altered ballots, on behalf of candidates, both black and white, who were primarily supported by white voters. But Sessions told the Senate that “no evidence was presented to us at that time of fraud by whites, at least anything credible.”

Justice Department policy in the 1980s supported bringing election cases “only when federal involvement is either necessary to vindicate paramount federal interests, or as a prosecutor of last resort to redress longstanding patterns of egregious electoral abuse.” Yet the Public Integrity Section of Ronald Reagan’s Justice Department approved Sessions’s case, saying it was not motivated by racial bias. Soon after the F.B.I. staked out the Perry County post office, the United States attorney general issued a statement that seemed related to Sessions’s prosecution: “Federal law prohibits political participants from intentionally seeking out the elderly, the socially disadvantaged or the illiterate for the purpose of subjugating their electoral will.”

When the Turners and Hogue went to trial, in June 1985, their supporters came in such numbers that entry to the courtroom in Selma required a ticket. Witness after witness among the 17 voters who testified said that while they’d received assistance from the Turners and Hogue to fill out their ballots, they’d done so willingly. The exceptions were members of a single family of six, who said their ballots were altered without their permission.