Both chambers of the Legislature are scheduled to meet on Monday morning for sessions that could bring fresh condemnations of the governor. As of Sunday evening, Lt. Gov. Justin Fairfax, who would succeed Mr. Northam if he resigned, had not been notified that the governor was stepping down.

Mr. Northam’s troubles began on Friday with the surfacing of a photograph on his medical school yearbook page, which showed a person in blackface posing with another in a Ku Klux Klan robe. The governor at first acknowledged that he was one of the figures in the image, and then denied it on Saturday, all while drawing widespread calls for his resignation. Until this episode, Democrats appeared to be on a steady roll in Virginia, a state that had increasingly become a source of strength for the party in major elections.

[Read: Ralph Northam rose to power quickly. His fall might be even faster.]

Since 2008, when Barack Obama became the first Democratic presidential candidate in more than four decades to carry the state, Virginia has shifted steadily leftward. For the last decade, both of the state’s senators in Washington have been Democrats. And more recently, the party has gained greater sway at the Capitol in Richmond.

Two years ago, Democrats picked up 15 seats in the House of Delegates, where they had been locked out of the majority for more than two decades. They are now two seats away from control in both chambers. The biggest prize in controlling the statehouse would be the power, under current law, to draw congressional and legislative districts after the 2020 census.

More power in the Legislature has already translated into significant policy wins for Democrats. Since Mr. Northam was elected in 2017, the party has achieved long-prized goals, like the expansion of Medicaid, and seized new credit for the state’s economic growth.