A Georgia judge allows another Saturday of early voting in the Senate runoff, a victory for Democrats.

ATLANTA — A Fulton County judge ruled on Friday that early voting in Georgia’s runoff election for Senate could be held on the Saturday after Thanksgiving, rejecting arguments that such a move was barred under state law.

The decision was a victory for Georgia Democrats, who had argued that the 2016 state law, which prohibits early voting on the second Saturday before a runoff if that day falls one or two days after a federal holiday, would unfairly disenfranchise voters who could not cast ballots during the runoff’s weekday early voting hours. These voters, who largely support Democrats, include students, low-income people and voters of color.

In a 10-page order, Judge Thomas A. Cox of the Superior Court of Fulton County ruled that the law did not explicitly apply to runoff elections and that Saturday voting “should be considered an essential component” of the election process, given that the window for runoff contests in Georgia is now shorter under the major voting law passed by state Republicans last year. This year’s runoff will be held on Dec. 6.

In the absence of Saturday voting, Mr. Cox ruled, the Democratic groups that filed the lawsuit and their constituents would “suffer immediate and irreparable harm.”

Georgia’s top election official, Secretary of State Brad Raffensperger, a Republican, had initially suggested that Saturday voting would be possible during the runoff, but then lawyers with his office said that it would conflict with the holiday weekend. The Saturday after Thanksgiving also falls one day after a state holiday that once commemorated the birthday of Robert E. Lee, the Confederate general, and is now observed without a specific focus.

Several Democratic groups, including the campaign of Senator Raphael Warnock, had sued on Tuesday to maintain Saturday voting, arguing that the secretary of state’s office had interpreted the law through too narrow a lens and that the move would disadvantage voters who tend to support Democrats. A representative for Mr. Raffensperger’s office did not indicate that it planned to appeal the decision, saying the attorney general’s office, led by Chris Carr, a Republican, would decide.

At a news conference on Tuesday, Mr. Warnock underlined his support for the lawsuit and encouraged county election officials to expand their voting hours.

In a statement on Friday, officials with the Democratic Senatorial Campaign Committee, the Democratic Party of Georgia and Mr. Warnock’s campaign called the judge’s ruling “a win for every Georgia voter, but especially for workers and students who will have a greater opportunity to make their voices heard in this election.”

They went on, “We look forward to counties announcing that they will provide Georgians the opportunity to cast their ballots on Saturday, November 26th.”

County election officials may allow Saturday voting at their discretion under the judge’s ruling. After Nov. 28, counties must allow early voting on weekdays until Dec. 2, the Friday before Election Day.