Virginia’s next governor will either be Democrat Ralph S. Northam or Republican Edward W. “Ed” Gillespie.

The choice of the competitors was made by 900,000 voters who trooped to the polls Tuesday to cast ballots in the primary elections for both political parties.

The turnout — representing about 16 percent of the state’s 5.4 million registered voters —was larger than observers expected.

Both men were the favorites to win their respective primary contests.

After four years of serving as the No.2 state leader to Gov. Terry McAuliffe, Lt. Gov. Northam had a much easier time winning the nomination than his GOP rival and appears more likely to have a united party behind him.

The 57-year-old pediatric neurologist and former state senator from Norfolk came into the Democratic primary leading in the polls.

And when the votes were counted, the results showed that he romped past his insurgent rival, one-term Congressman Tom S. Perriello, capturing 54 percent of the more 540,000 votes cast in the Democratic primary.

Mr. Perriello, who jumped into the race in early January in the wake of President Trump’s election, carried the endorsements of two progressive icons, U.S. Sens. Bernie Sanders and Elizabeth Warren.

But he was hard-pressed to differentiate himself from Lt. Gov. Northam, who came into the race with a fatter campaign war chest, an equally strong progressive record on issues ranging from health care to gun control and the endorsements of virtually every elected official in Virginia.

Lt. Gov. Northam is eager now to turn to the general election set for Nov. 7 and keep Virginia’s top elected office in Democratic hands.

“We’re ready to fight,” Lt. Gov. Northam told supporters at a post-election rally in Northern Virginia as he prepared to lead the Democratic ticket that will include newcomer Justin E. Fairfax as the lieutenant governor nominee and incumbent Attorney General Mark R. Herring, who sought a second term rather than challenge Lt. Gov. Northam.

In a sign of a unified party, Mr. Perriello already has embraced the ticket and pledged his support.

They will take on a Republican ticket representing a party that appears more splintered.

The Republican vote certainly confirmed the split between those who ardently support President Trump and those who do not.

Like Lt. Gov. Northam, Mr. Gillespie, 55, a former Republican National Committee chairman and counselor to former President George W. Bush, came into the three-way GOP gubernatorial primary expecting to win, having picked up most of the endorsements and financial support.

However, Mr. Gillespie, who lost to U.S. Sen. Mark Warner in 2014, barely eked out a victory over Corey A. Stewart, President Trump’s one-time Virginia campaign director.

More than 366,000 people voted in the contest. Mr. Gillespie, who drew scorn from Mr. Stewart for failing to embrace President Trump in his campaign, gained 43.76 percent of the vote to 42.5 percent for Mr. Stewart and 13.75 percent for the state Sen. Frank W. Wagner, according to unofficial results.

Mr. Stewart grudgingly conceded Wednesday to Mr. Gillespie. The state Department of Elections by then was reporting that with all precincts and absentee votes counted, Mr. Gillespie had a 4,609 vote lead over Mr. Stewart — a 1.26 percentage point margin that would bar a recount.

However, Mr. Stewart, an anti-immigrant advocate who defends Confederate statuary in the state, refused after his loss to endorse Mr. Gillespie and hinted at plans to run against U.S. Sen. Tim Kaine, who will be seeking re-election next year.

“There is one word you will never hear from me, and that’s ‘unity,’ ” a defiant Mr. Stewart told supporters at his post-election rally in Prince William County.

His words suggest Mr. Gillespie will have a harder time rallying his party to support the ticket he will lead.

The ticket also includes state Sen. Jill H. Vogel, who is seeking to become the first female lieutenant governor in Virginia, and GOP attorney general nominee John Adams, a Richmond corporate attorney and former associate White House counsel.

Ahead for voters is likely to be an expensive and hard-fought race for the South’s only open governorship. New Jersey is the only other state electing a governor in November.

Once reliably Republican, Virginia is the only Southern state that Democrat Hillary Clinton carried in 2016. The commonwealth has now gone Democratic in three successive presidential elections.

Also, Virginia is among just three of the 14 Southern states with a Democratic chief executive, the others being West Virginia and Louisiana. Three of the commonwealth’s last four governors have been Democrats.

Voters will decide Nov. 7 whether the governor’s office remains a Democratic stronghold in an era in which the General Assembly has Republican majorities in the House and Senate.