Georgia voters enter and proceed through the process of "Advanced Voting" at the Cobb County Board of Elections and Registration office in Marietta, Ga., on Oct. 15. (Melina Mara/The Washington Post)

With an increasingly competitive and closely-watched Senate race on the line, an Atlanta courtroom will be the focus of a key voting rights dispute Friday that could make it harder for Democrats to pick up an open U.S. Senate seat on Election Day.

At issue is the fate of approximately 40,000 registration applications submitted with the help of a new voter registration group led by the state's Democratic House minority leader. She's joined in her legal battle by the NAACP and other civil rights groups, who are challenging the Republican secretary of state over whether his office has adequately processed ballot applications.

Any irregularities could spoil Democratic attempts to turn out base voters and help Democratic Senate candidate Michelle Nunn defeat her Republican opponent, David Perdue, or Democratic State Sen. Jason Carter in his bid to unseat Republican Gov. Nathan Deal. Polls show that both races remain close. If no candidate earns 50 percent on Election Day, the governor's race faces a December runoff. A Senate runoff would be Jan. 6.

Demographers and political observers agree that there are roughly 800,000 eligible but unregistered voters in Georgia -- an ethnically diverse mix of Latinos, African Americans and Asian Americans and whites -- many of whom have moved into the Peach State in recent years.

The New Georgia Project, a group launched this year by Stacey Abrams, Georgia's Democratic House minority leader, registered nearly 86,000 new voters before the deadline this month. Partner organizations, including the NAACP, say they registered another approximately 30,000 new people.

"If you want to get people engaged, if you want people to understand the issues being made on their behalf, there’s no more pertinent time than during an election," Abrams said in a recent interview with The Washington Post. She insists that her new organization is nonpartisan, but the group's efforts are bound to benefit Democrats -- a point she didn't deny.

"Both parties have both an opportunity and obligation to market themselves to these registered voters," Abrams said. "I think our pitch is better. I think Democrats have a stronger message that will resonate with this community."

Despite her party affiliation and senior political role, Abrams insisted later that "When we’re engaged in voter registration, there should be no question that it’s being done to the letter of the law."

That point has been is in dispute.

State election officials, led by Secretary of State Brian Kemp, said earlier this year that they had reviewed registration applications submitted by the group and found several discrepancies. Aides in Kemp's office said they found 513 names of deceased Georgians; 1,637 records that matched the names of ineligible convicted felons; 2,124 records with no valid birth year; and 2,195 with invalid or out-of-state ZIP codes.

Those figures don't necessarily suggest wrongdoing or fraud -- especially since there's no evidence that any of those registrations in questions were intended, let alone used, to actually vote, and may reflect on the normal churn of life and and backlogged bookkeeping by the state.

Ultimately, Kemp's office announced that a review had found less than 1 percent of voter registration forms turned in by the groups -- fewer than 100 -- were actually fraudulent or suspicious. Of those, just 25 were confirmed forgeries, though there was no evidence anyone had planned to use those registrations to cast fraudulent votes.

Leaders of the New Georgia Project, the NAACP and other groups had worked with Kemp's office to ensure that their registration project went smoothly. But amid delays in processing the applications, they filed suit against Kemp's office and five counties -- Chatham, Clayton, DeKalb, Fulton and Muskogee -- alleging that a search of state databases had failed to provide evidence that roughly 40,000 of the registration applications they had submitted had been processed.

The counties encompass areas around Atlanta and the Democratic strongholds of Columbus and Savannah.

This week the activists settled out of court with DeKalb County, one of the Atlanta-area counties, but the suit will proceed Friday against Kemp and the other counties.

Kemp recently called the lawsuit "frivolous and totally without merit." He called allegations that there are 40,000 missing or unprocessed applications "absolutely false" and said that all counties have processed registration applications.

"It is time for the New Georgia Project and others to stop throwing out random numbers and baseless accusations and let the counties continue to do their jobs," he added.

Proof that any voter has been turned away due to an unprocessed ballot application hasn't materialized. When asked repeatedly in recent days for examples of voters turned away due to fraudulent or nonexistent ballots, representatives for New Georgia Project and the NAACP told The Post that they didn't know of any.

But Francys Johnson, president of the NAACP's Georgia conference, said his organization has been sparring for several years with Kemp's office. He accused the Georgia NAACP of ballot fraud in 2010 and in subsequent years has sought to curtail the length of early voting. Johnson said that much of the dispute is rooted in partisan politics and a fear among Republicans that Democrats could be on the verge of retaking control of the state after more than a decade out of power.

Noting that Kemp had once run for state agriculture secretary, Johnson suggested that "maybe he would have been better at teaching people how to shovel manure instead of running elections."

If Democrats can make gains this year, "It puts another state in the deep South on the table for 2016," he said. "That’s big, because the notion of the 'Solid South' has been a huge issue for progressive politics in this country. That is a notion that is going to go by the wayside when you have a state as big and important as Georgia in play."