To some Democrats, Mr. Fairfax’s alleged conduct is the most serious because he is the only one of the three accused of a crime. But that does not make the political quandary any less torturous at a moment when the party’s 2020 presidential primary is getting underway with more black and female candidates than have ever run for the White House.

“To show a firm grasp of the obvious, the optics would be difficult and the substance would be difficult,” said State Senator J. Chapman Petersen, who is white, about how it would look if Mr. Northam and Mr. Herring remained in office while Mr. Fairfax was exiled.

Women and African-Americans have never been more politically powerful: The Democrats’ 40-seat win in the House midterm elections in November, as well as their 2017 triumph in the top Virginia races, was powered in no small part by those voters. And with Republicans barely hanging on to their legislative majority in the Virginia Capitol, Democrats were counting on the same two blocs to propel them to victory in this fall’s election of all 140 delegates and state senators.

Ultimately, some Democrats here said, they must begin the process of emerging from the wreckage that is the executive branch of Virginia state government by turning to what is perhaps their most loyal constituency: black women.

And barely hours after Ms. Watson came forward on Friday saying she was raped by Mr. Fairfax in 2000 when they were students at Duke University, several senior Virginia Democrats began making the case that should Mr. Northam continue his refusal to resign, he ought to appoint State Senator Jennifer McClellan to replace Mr. Fairfax if he quits or is impeached. (It is not certain that Mr. Northam could appoint any successor to Mr. Fairfax, scholars said, because of conflicting provisions and interpretations of the Virginia Constitution and state law.)

Ms. McClellan, who is black, is a longtime Richmond legislator who was already thought to have statewide ambitions and has a close relationship with Senator Tim Kaine.

“Jennifer would make an exceptionally good lieutenant governor,” said C. Richard Cranwell, a former state Democratic chairman and legislator, when asked about the senator.