An Atlanta judge ruled on Friday that Fani T. Willis, the Fulton County district attorney, could continue leading the election interference prosecution of former President Donald J. Trump and his allies in Georgia, but only if her former romantic partner, Nathan J. Wade, withdrew from the case.

The highly anticipated ruling by Judge Scott McAfee of Fulton Superior Court cut a middle path between removing Ms. Willis for a conflict of interest, which defense lawyers had sought, and her full vindication. The judge sharply criticized Ms. Willis for dating Mr. Wade, whom she hired as a special prosecutor on the case, calling it a “tremendous lapse in judgment.”

Hours after the ruling, Ms. Willis said that Mr. Wade had offered his resignation, and that she had accepted it.

Judge McAfee had rejected a defense claim that the relationship had raised an actual conflict of interest by giving Ms. Willis a financial stake in the case. But he found that it had raised “a significant appearance of impropriety” that needed to be addressed.

Disqualifying Ms. Willis and her office from the case was not necessary, the judge said, when “a less drastic and sufficiently remedial option is available.” But he concluded that “the prosecution of this case cannot proceed until the state selects one of two options.” Either Ms. Willis could have stepped aside with her office, including with Mr. Wade, or he had to leave.