RICHMOND, Va. — Lt. Gov. Justin E. Fairfax of Virginia faced a wave of demands for his resignation on Friday night — and the threat of impeachment proceedings — after a second woman came forward to accuse him of sexual assault.

His detractors included an increasing number of fellow Virginia Democrats, including Senator Tim Kaine, at least seven members of the state’s congressional delegation and former Gov. Terry McAuliffe. Some raised pointed questions about whether he remained fit to succeed Gov. Ralph Northam, who has been under pressure to resign over his acknowledgment that he once wore blackface.

Mr. Fairfax, who until this week was a rising star in Virginia politics and only the second black man ever elected to statewide office, denied any wrongdoing and vowed not to quit. But his political standing appeared to be in a free fall by Friday night, when the Democratic caucuses in the state House and Senate said that Mr. Fairfax needed to quit.

“Due to the serious nature of these allegations, we believe Lt. Gov. Fairfax can no longer fulfill his duties to the Commonwealth,” the caucuses said in a statement. “He needs to address this as a private citizen. The time has come for him to step down.”