Lt. Gov Ralph S. Northam, the Democratic nominee for governor in Virginia, at a campaign event in Chesterfield, Va., on Tuesday. CreditSteve Helber/Associated Press

ARLINGTON, Va. — Ralph S. Northam, Virginia’s lieutenant governor, claimed the Democratic nomination for governor on Tuesday, The Associated Press reported.

A mild-mannered physician and Army veteran, Mr. Northam, 57, fended off Tom Perriello, a former congressman whose insurgent candidacy gave the state’s Democratic establishment a scare. He begins the general election as the favorite against the Republican candidate, Ed Gillespie.

A Virginia Gentleman

What both scares Republicans and worries Democrats is that Mr. Northam could have been a candidate for Virginia governor just as easily 30 years ago as today. With the accent of his youth on the state’s rural Eastern Shore, a diploma from the Virginia Military Institute and a courtly Southern bearing, Mr. Northam is not exactly the typical Democrat of 2017.

That makes some Republicans nervous because they believe he could pull votes from his fellow veterans and other conservative-leaning constituencies. But it also troubles some Democrats, who are unsure that Mr. Northam — who twice voted for George W. Bush — will be able to energize the party’s ascendant liberal base.

Progressive Bona Fides

Even though Mr. Perriello sought to outflank him on the left, Mr. Northam won in part because of his strength with crucial liberal voter blocs. Mr. Northam, who often attends a Baptist church with a largely black congregation, overwhelmed Mr. Perriello in Virginia’s most heavily African-American jurisdictions. And his advocacy as a state senator and lieutenant governor for teachers, gun control, abortion rights and a smoking ban in restaurants won him support that might have otherwise gone to a more progressive candidate.

Four More Years

Virginia bars governors from running for re-election, the last state in the country with such a restriction, but Mr. Northam made no secret that he was effectively running for Gov. Terry McAuliffe’s second term. He featured the governor, who is highly popular among Democrats around the state, in television ads and promoted the economic progress the state has made in the last four years. His wager in the primary, and now in the general election, is that Virginia voters are happy with their state government and will support continuity.

Turn It Up

Mr. Northam scarcely mentioned President Trump’s name at the outset of the primary. But he soon saw how hungry Democratic voters were to confront the president, and he began calling Mr. Trump “a narcissistic maniac.” The insult made its way into his TV ads. But high-decibel attacks do not come easy to Mr. Northam, a pediatric neurologist, and he may have to be nudged to take it to Mr. Trump in the fashion many Virginia Democrats are expecting.