A federal judge on Thursday ordered the state of Georgia to extend until May 21 the deadline to register to vote in a closely watched special congressional election taking place next month.

The decision will affect the race between Democrat Jon Ossoff and Republican Karen Handel in Georgias 6th Congressional District. Ossoff has seen an unexpected amount of support in the district long controlled by Republicans , and many are looking to the race as a bellwether of anti-Donald Trump political energy in the early months his presidency.

Five civil rights and civic engagement groups sued the state last month, claiming that it was violating the National Voter Registration Act. The law says that a state may set a cut off for an election no sooner than 30 days before it occurs.

Georgia met that requirement for an April 18 election to fill the House seat vacated by Tom Price, Trumps pick for health and human services secretary. But because no candidate in the race received 50 percent of the vote, Ossoff and Handel advanced to a runoff.

The state argued that the runoff was a continuation of the April election. Because of that, the state said, anyone who had not registered by March 20 could not vote in June. The plaintiffs argued that violated the NVRA, which clearly says the 30-day rule applies to runoff elections as well.