Georgia won’t permit newly registered voters to participate in the June 20 runoff election for the state’s Sixth Congressional District — a restriction that a new lawsuit argues is illegal.

The National Voter Registration Act requires that states allow eligible adults to register to vote at least 30 days before an election. In Georgia, voters can register at least 30 days before a primary. However, if people want to vote in the general election, they need to have registered before the primary.

For the June 20 special election, in other words, the registration window effectively closed March 20. That has big implications for a high-profile race to replace Tom Price, currently serving as Trump’s secretary of health and human services. The Sixth Congressional District has long been reliably Republican, but Democrat Jon Ossoff, backed by an outpouring of national support, got 48 percent of the vote in the primary Tuesday night. The result has made the district a prime target for a Democratic victory.

Democrats estimate that there are more than 800,000 unregistered, eligible voters in the state — mostly people of color. Before the midterm elections in 2014, the state was estimated to have about 664,000 more registered Republicans than registered Democrats.

The Lawyers’ Committee for Civil Rights Under Law sued the state, arguing that would-be voters should have until May 22, 30 days before the runoff, to register. That means the plaintiffs in the lawsuit, which include the Georgia chapter of the NAACP and the Atlanta chapter of Asian Americans Advancing Justice, have about one month to make their case and register any new would-be voters that could become eligible.

The office of Georgia Secretary of State Brian Kemp vowed to fight the lawsuit, saying that the law in question has been in place since Democrat Cathy Cox was secretary of state in the late ’90s and early 2000s. Kemp himself has called it a “completely political effort” to attack him.

This isn’t the first time Kemp and his office have been accused of violating voters’ rights. In 2014, when more than 40,000 individual voter registrations reportedly went missing, the New Georgia Project and the Georgia chapter of the NAACP, who registered the would-be voters, sued the secretary of state, claiming that the registrations were from “members of the underrepresented classes of voters.” The case was ultimately dismissed because the court found that there was still time before the midterms that year for the missing registrations to reappear.

Last month, Kemp announced he will be running for governor in 2018, to succeed the term-limited incumbent, Gov. Nathan Deal. Many anticipate that Stacey Abrams, the Georgia House minority leader, will run too. Abrams is also the founder of the New Georgia Project. If this year’s special election has become something of a referendum on early Trump-era Democratic resistance, it’s looking like the 2018 gubernatorial race could become a referendum of sorts on Georgia voting rights.