Much of that rise reflects purges in many of the places once subject to the preclearance requirements of the Voting Rights Act. Those requirements meant that places with a history of discrimination had to seek approval from the federal government or courts before they could make changes to voting laws.

But in 2013, the Supreme Court knocked down the preclearance provision of the Voting Rights Act. Two million more people were booted from registration lists between 2012 and 2016 in jurisdictions once covered by preclearance than would have been kicked off if purge rates in those areas continued at the same rate as jurisdictions that hadn’t been subject to preclearance.

And as regions that used to be covered by the preclearance provisions increased their purge rates, we found, so too did the number of people who showed up to vote at their polling place but were unable to cast a regular ballot. This suggests that voters had been wrongly removed from the rolls.

Our concern isn’t limited to Southern states. We found that over the past five years, four states have engaged in illegal purges (Florida, New York, North Carolina and Virginia), and another four states (Alabama, Arizona, Indiana and Maine) have written policies that violate the National Voter Registration Act, which helps protect against wrongful purges.

In particular, Alabama, Indiana and Maine have policies that disregard the federal requirement to allow at least two elections of nonvoting before tossing voters from the rolls. Instead, all three states allow the use of a problematic multistate database called Crosscheck to conduct immediate purges. Crosscheck purportedly compares registration lists across states, but it might flag a voter if only name and date of birth match, which is not precise enough to prevent mistakes.

Crosscheck has led to unfair purges elsewhere. In 2013, Virginia used Crosscheck to purge some 39,000 voters. County officials received rosters of potential Crosscheck “matches” without checking for accuracy, nor did they have sufficient time to conduct a thorough review. In some counties, error rates were as high as 17 percent.