Mr. Northam, 59, now teeters precariously on the top rung of a political ladder that he ascended in just 10 years, aided by his biography as a pediatrician, an Army officer and a rural Virginian in a state where Democrats have little strength outside cities and suburbs.

His low-key manner, which on the campaign trail manifested as a doctor’s reassurance, appeared to work against Mr. Northam at a news conference on Saturday at the Executive Mansion in Richmond, suggesting a kind of obliviousness. At one point, he seemed on the verge of demonstrating Michael Jackson’s moonwalk — which he said he once did in a dance contest while wearing shoe polish to darken his face — until his wife, Pam, who was standing next to him, whispered, “Inappropriate circumstances.”

Mr. Northam was recruited into politics in 2007 by Levar Stoney, then an official with the state Democratic Party and now the mayor of Richmond. He gave Mr. Northam the nickname “Blue Crab” because of his roots on the state’s Eastern Shore, a center of crab and oyster fishing and farming. Mr. Stoney, an African-American, was among the Democratic leaders who called for the governor to quit.

Mr. Northam was raised on an inlet of Chesapeake Bay, the son of a judge and heir to a long lineage in the racially diverse Accomack County. Today, the small town where he grew up, Onancock, is reached past cotton fields, barns with fallen roofs and the rusty rails of a moribund railroad. Antique stores and real estate offices line the handful of commercial blocks along Market Street.

The governor attended the Virginia Military Institute for his undergraduate studies and then Eastern Virginia Medical School, which is in Norfolk.