A federal judge on Thursday ordered the state of Georgia to temporarily re-open early voter registration as a tense special election approaches.

U.S. District Judge Timothy Batten ruled in favor of the Lawyers Committee for Civil Rights Under Law, an advocacy group that accused Georgia of violating federal law by reducing the registration period ahead of the 6th District congressional runoff on June 20.

Voters will now have until May 21 to sign up, after the state tried to cut off registration after March 20.

"Would-be voters in Georgia's 6th Congressional District were denied a full and fair opportunity to register and vote under the prior law," said the committee's president and executive director, Kristen Clarke, in a statement following the ruling. "Today's decision extending the voter registration deadline helps ensure that eligible voters will be able to participate in the upcoming runoff election, and in all future runoff elections for federal office."

"States like Georgia must stop taking actions that suppress the rights of voters," Clarke said.

Democratic newcomer Jon Ossoff will face off with Republican and former Georgia secretary of state Karen Handel in the race to fill the seat left behind by Health and Human Services (HHS) Secretary Tom Price.

The race is seen as a bellwether for the political resistance to President Donald Trump. An internal poll conducted by the Atlanta Journal-Constitution on Tuesday found the candidates "locked in a dead heat."

Ossoff narrowly missed getting 50 percent of the vote in a crowded primary last month—but he has significant support from Democrats, who are volunteering and donating to his campaign in record numbers, and Politico reported Thursday that he has "quietly attracted a small but potentially pivotal share of Republican support in his special election race, according to a new analysis of the April 18 primary."

An analysis conducted by the AJC last month found that voter registration in the district increased by 2.5 percent after Trump's election.