Third of seven parts.

SPARTA, Ga. - The cleansing of America's voter registration rolls occurs every two years and has become a legal battleground between politicians who say the purges are fair and necessary, and voting rights advocates who contend that they discriminate.

Voting rights groups repeatedly have challenged states' registration purges, including those in Ohio, Georgia, Kansas, and Iowa, contending that black, Latino, poor, young and homeless voters have been disproportionately purged. In Florida, Kansas, Iowa, and Harris County, Texas, courts have ordered elections officials to restore thousands of voters to the registration rolls or to halt purges they found discriminatory.

The 1993 National Voter Registration Act mandates that state and local elections officers keep voter registration lists accurate by removing the names of people who die, move or fail in successive elections to vote. Voters who have been convicted of a felony, ruled mentally incompetent, or found to be noncitizens also can be removed. The U.S. Election Assistance Commission reported that 15 million names were scrubbed from the lists nationally in 2014.

News21 analyzed lists of nearly 50 million registered voters from a dozen states, and seven million more who were removed over the last year. By comparing voter registration and purge lists against U.S. Census data, News21 found no national or statewide pattern of discrimination against voters based on race, ethnicity, poverty, age or surname.

But the data did show that purges disproportionately affected minority or low-income voters in certain communities, and white voters in others. In Cincinnati, poverty rates and voter removals appeared interrelated, while race appeared to affect the removal of voters in rural Hancock County, Ga. In Vermillion County, Ind., a shrinking population accounted for large numbers of white registered voters being removed from the rolls.

David Becker, the director of elections initiatives at the nonpartisan Pew Center for Charitable Trusts in Philadelphia, said the national pattern mades sense to him. "I think you're finding exactly what I've found in my experience," he said.

But Michael McDonald, a University of Florida political scientist who created the U.S. Election Project to track election and demographic data, said he was surprised by the lack of a pattern in the national findings, although hidden patterns of discrimination can be found on the local level. "Sometimes you have to look under the hood," McDonald said.

Local election officials clean up voting registration lists under state laws governing who should be removed from the lists. Some states bar felons from voting, but they vary dramatically on which crimes disqualify voters or how quickly they can be reinstated. Some, but not all, states require elections officials to mail warning notifications to voters whose names are slated for removal. States also vary on how many elections a voter can sit out before being classified as "inactive" and later culled.

Most states work together or with the federal government to compare voter registration lists to stop people from voting in two states in the same election. One in eight Americans move every year and people often remain registered in two places. Few cancel old registrations.

The NVRA directed local elections officials to mail reminders to registered voters who had stopped voting in their jurisdictions to verify whether voters still live where they are registered and to establish who has moved. Those who fail to respond are placed on an inactive list and cut from voter registration rolls after one more missed election.

In 2005, Kansas created the Interstate Crosscheck System, in which 30 states compare registration lists for duplicate entries. When duplicate listings are found, state elections notify each other or local elections offices. It's up to those local offices to remove those names from the voter rolls. Some do, some don't.

Pew created another name-matching tool in 2012. Its Electronic Registration Information Center has 21 state partners and compares more extensive records than Crosscheck.

At least 16 states also work with the U.S. Department of Homeland Security to apply its immigration database, the Systematic Alien Verification for Entitlements (SAVE), to confirm the citizenship of voters.

Nationwide, there are around 10,500 state, county and local elections offices. Resources for managing elections vary widely from large urban counties to small rural communities. That accounts for much of the variety in how registration rolls are maintained.

"Often it's an administrative or technical issue," Becker said. "People are not deliberately trying to remove people. They have a very hard job. They have a legitimate interest to make sure we are not spending taxpayer money to send information to people who are not voters."

Vermillion County, Ind., typifies why cleaning up voter registration rolls is necessary.

In the two years leading up to the 2014 election, County Clerk Florinda Pruitt scrubbed nearly four registrants out of every 10 in the small western county of corn and soybean fields.

Most of the 1,191 voters were removed in the current election cycle because of death or change of address, News21 found.

"We really want people to vote. We really work at it. But if you don't want to vote, don't register. Don't clog our system," Pruitt said, adding no voter there had ever challenged a removal.

In Sparta, the seat of Hancock County, Ga., race appeared to play a critical role in a voter purge in 2015.

Last year, Sparta, whose population of around 1,300 is about 80 percent black, was about to choose between white and black candidates for mayor. The majority-white Hancock County Board of Elections sent teams block by block through Sparta looking for residents to vouch that their neighbors still lived where they were registered to vote.

On the say-so of neighbors, 174 voters were listed for removal, including Dion Hubert and his brother, who'd been taking care of their ailing mother across town. Hubert, 29, skipped work to attend a hearing at the courthouse to petition the elections board. He prevailed, but many others never showed up.

"They called name by name and if they didn't answer, they'd just cross them off the list," Hubert said. "They got a kind of excitement out of what they were doing, by taking people's names off the list."

Almost all voters purged from the rolls were black, according to a lawsuit filed later by the NAACP. The Hancock County Board of Elections declined interviews, but the county denied every allegation in court without elaboration. The case has not yet gone to trial.

When Hamilton County, Ohio, where Cincinnati is the county seat, removed 75,000 voters this year, nearly half, in some neighborhoods far more, were purged because of "nonresponse."

In central Cincinnati, where the poverty rate in the Clifton Heights neighborhood is double the city's average, Hamilton County officials so far removed 27 percent of the voters. Six miles west, in the almost exclusively white Cheviot neighborhood, where the poverty rate is half that of the region, the county purged 9 percent of voters.

Conservative activist groups view inaccurate registration rolls as a problem for democracy. The conservative American Civil Rights Union has sued eight counties to cleanse the rolls. A year ago the Public Interest Legal Foundation, which litigates on behalf of the ACRU, said it sent warning letters to 141 more. In January, the legal foundation said it threatened to sue 30 counties.

"Across the country, hundreds of other counties have more registered voters than people alive. If they don't clean up their rolls, they risk litigation," ACRU Chairwoman Susan Carleson wrote on the group's website. "Every time an illegal voter casts a ballot, it steals someone else's legal vote. The goal is to ensure the integrity of the voting process."

Sean Young, senior staff attorney for the Voting Rights Project at the liberal-leaning American Civil Liberties Union, said the renewed focus on purging registration lists is no coincidence.

"These groups are trying to use the statutes in the NVRA as a tool to disenfranchise voters," Young said. "Their efforts have certainly been reinvigorated, in direct response to record numbers of registrations of African-Americans."

Emily L. Mahoney, Hillary Davis and Jimmy Miller of News21 contributed to this article.