Raphael Warnock’s campaign sued Georgia on Tuesday after the state said it would not offer Saturday early voting for the closely watched runoff in which Warnock is seeking re-election to the US Senate.

The suit challenges the state’s interpretation of a law that would prohibit early voting on the Saturday following Thanksgiving. The day after Thanksgiving is also a state holiday in Georgia, originally to commemorate Robert E Lee, the Confederate civil war general. In 2015, state officials dropped Lee’s name and started recognizing the day simply as a “state holiday”.

Lawyers for Warnock’s campaign argued that the voting prohibition only applies to primary and general elections, not runoffs, which have a much shorter voting period. Last year, Georgia Republicans passed a law shortening the runoff period from nine weeks to four. But the shortened runoff period is coming into conflict with the state law that bars early voting around holidays.

The state “misreads” and “cherry-picks” provisions of the law that do not apply to runoffs, lawyers for Warnock’s campaign wrote in a complaint, which was joined by the Georgia Democratic party and the Democratic Senatorial Campaign Committee. “The secretary’s insistence that counties may not hold advance voting on November 26 therefore has no support in the law and conflicts with [the statute’s] requirement that counties begin advance voting for the December 6 runoff as soon as possible,” it reads.

The suit asks a judge to declare that state law does not prohibit counties from offering early voting on the Saturday after Thanksgiving and to order the state not to take any action blocking them from doing so.

Georgia law says counties may start early voting “as soon as possible” after the state certifies results from the general election, with a mandatory period from 28 November to 2 December. State law also bars Saturday early voting on the Saturday before a runoff, 3 December.

Georgia’s secretary of state, Brad Raffensperger, initially said he believed some counties would offer early voting on Saturday, before his office reversed course and said it was prohibited.

“It’s not our choice. It’s literally in black-letter law that the Saturday following a state holiday cannot be used for early voting,” Gabriel Sterling, an interim deputy secretary of state, told the Atlanta Journal Constitution. “We all thought there was going to be Saturday voting until we looked at the law really closely.”

Gerald Griggs, the president of the Georgia NAACP, said a Confederate holiday should not block voting.

A coalition of civil rights groups separately sent a letter to all of Georgia’s 159 counties on Tuesday urging them to offer at least three additional days of early voting from 7am to 7pm, which is allowed at the counties’ discretion under Georgia law. The coalition urged the counties to offer voting on the Tuesday and Wednesday preceding Thanksgiving as well as the Sunday after the holiday.

“If you only offer advance voting on the five days required by statute (which, as noted above, is limited to weekdays), there is a significant risk that many voters will be unable to participate due to obligations during the workday,” lawyers for the NAACP Legal Defense Fund, the Southern Poverty Law Center and the ACLU of Georgia wrote in the letter. “The lack of weekend or evening voting options is especially concerning for voters of color, who may be less able to take time off from work to vote.”

Hillary Holley, the executive director of Care in Action, the political arm of the National Domestic Workers Alliance, tweeted on Tuesday that Fulton county, home to Atlanta and the state’s most populous county, had agreed to start early voting on the Sunday after Thanksgiving. She also said Gwinnett county, one of the largest and most diverse counties in the state, had also agreed to start early voting on Sunday.