Voters cast ballots during the early voting period at C.T. Martin Natatorium and Recreation Center on October 18, 2018 in Atlanta, Georgia.

In its race for governor, Georgia is now conducting a state-of-the-art experiment illuminating the partisan debate on whether voting restrictions tilt American elections.

As it happens, laboratory conditions are ideal.

The Democratic candidate is Stacey Abrams, an African-American former legislator who opposes recent restrictions as impediments to minority voting. She faces Brian Kemp, the white Republican who oversees those restrictions as Georgia's Secretary of State.

Kemp rejects her call to resign in the name of election impartiality, vowing to remain on the job even if the race ends up close enough for a recount. A new NBC News/Marist poll today suggests that could happen: among registered voters, Abrams and Kemp stand deadlocked, 47 percent to 47 percent.

The race plays out against a national backdrop of disquiet about democracy itself. As America grows more diverse and polarized, President Donald Trump and Republican officials have increasingly embraced steps to prevent what they call "voter fraud."

In reality, there's so little evidence voter fraud exists at all that Trump's appointed voter-fraud commission collapsed. Backed by independent experts, Democrats say the GOP's principal goal is limiting ballots cast by Democratic-leaning black, Latino, young and low-income voters.

"These laws have been pushed in recent years by Republicans, and the hardest hit have been people of color and young people and poor people," says Michael Waldman, president of the Brennan Center for Justice at New York University Law School. "Restricting the vote appears to be strategic."

The restrictions range from requiring government-issued photo identification to vote, to delaying voter registration if application information differs from government databases, to limiting voting times and locations. What remains unclear is how much they actually deter voting.

Two political scientists found that Wisconsin's voter ID law deterred thousands of voters, disproportionately poor and African-Americans from casting ballots in a state Trump narrowly carried in 2016. Another study found similar effects nationally, especially among Hispanics.

"Our research shows that these laws lower minority turnout and benefit the Republican Party," a team of political scientists from Bucknell, Michigan State and University of California-San Diego wrote in the Washington Post.

But other researchers don't find the evidence persuasive. And strategists note that restrictions can enhance motivation to vote among those who feel targeted, and bolster efforts by others to help them.

In 2012, for example, after Georgia narrowed its pre-election early voting window, election officials in majority-black DeKalb County added more early-voting locations. The proportion of votes cast early did not fall.

This year, amid a tight race for Kansas governor, election officials in majority-Hispanic Dodge City moved the only polling place outside city limits, more than a mile from the nearest bus stop. The advocacy group Voto Latino promptly announced a partnership with ride-share company Lyft to transport local residents to vote.

In Georgia, voting-rights advocates have accused Kemp for overly aggressive "purges" of voting rolls, including more than 100,000 people who hadn't voted in earlier elections. They've also challenged the delay of voter registrations for 53,000 more people, most of them black, under an "exact match" law requiring applications to mirror driver's license or Social Security data information.

As proof of Kemp's intentions, Democrats cite leaked audio of a recent closed-door talk with donors. Kemp told them that a pro-Abrams absentee ballot program "continues to concern us, especially if everybody uses and exercises their right to vote."

The subject dominated their televised debate this week. Abrams accused Kemp of using "an atmosphere of fear" to deter voting, he responded that "it's never been easier to register to vote."

Nationally, Kemp is right. The long-term trend in American elections is toward making it easier for time-stressed American voters to cast ballots.

Through the rise of early voting and no-excuse-required absentees balloting, roughly one-third of votes were cast before Election Day in 2014. This year, the Brennan Center reports a trend in states toward automatic voter registration when people interact with government agencies such as driver's license offices.

Next door to the Georgia clash, Floridians are considering a major expansion of voting rights. A Nov. 6 ballot measure restoring the right to vote for 1.5-million felons who completed their sentences enjoys bipartisan support in the polls.

"In the long run there's a lot of progress being made,' said the Brennan Center's Waldman. "The question is whether the bipartisan forces for reform are faster than the partisan forces of disenfranchisement."