Lt. Gov. Ralph S. Northam said Tuesday that he intends to run for governor in 2017, setting up a possible primary battle with another likely candidate, Attorney General Mark R. Herring.

In a state where the governor cannot serve back-to-back terms, Northam and Herring were both widely assumed to have their eyes on the governor’s mansion from the moment in 2013 when they won their current offices on a ticket with now-Gov. Terry McAuliffe (D). But neither had definitively said he planned to run — until now.

In the past few weeks, Northam has begun putting the word out, privately informing McAuliffe, Democratic leaders and supporters that he plans to run, the lieutenant governor confirmed Tuesday during an interview with The Washington Post.

“I’m planning for the next step — planning to run for governor,” Northam said.

Several Democrats said they welcomed the news that Northam — a Virginia Military Institute graduate, Gulf War veteran and pediatric neurologist who hunts, fishes and speaks with the folksy drawl of an Eastern Shore native — was entering the race. They said his biography and collegial manner could play well across an increasingly polarized state, where Democrats have dominated Northern Virginia and other urban centers but have little following elsewhere.

A few Democrats expressed reservations, however, about the strain on the party that would come from a primary fight between Northam and Herring (D), who is widely expected to run but has yet to declare his intentions. Several expressed hope that one of the two Democrats would decide to step aside.

“I think we’re doing a lot of good work for the people of Virginia, and I’ll make decisions about future elections later,” Herring said via e-mail.

Northam said that he, too, is focused on his current job, which involves presiding over the Senate and working with McAuliffe on economic development, mental health issues, early childhood development and the redevelopment of Fort Monroe in Hampton.

“The best way to run or apply for a new job is to have a good record,” he said. “And that’s what I want to work very hard on in the next three years.”

Northam’s early, albeit unofficial, entrance into the race is an aggressive move on the part of someone generally known for his low-key style. It could be a bid to catch up with Herring, whose higher-profile perch has often put the attorney general in the headlines.

Herring has used his office to buck the state’s gay marriage ban, declare some illegal immigrants eligible for in-state tuition and advance other liberal causes that could endear him to Democratic primary voters but also complicate a general election bid.

As lieutenant governor — a part-time job in Virginia — Northam’s biggest power is to break tie votes in the Senate. He has taken liberal positions without becoming the lightning rod that Herring has for some Republicans, who offered several bills this session seeking to rein in the attorney general’s powers.

Northam’s early move also was seen as evidence that the soaring cost of running for governor in Virginia is forcing campaigns to start ever sooner. The 2013 battle between McAuliffe and then-attorney general Ken Cuccinelli II (R) cost about $60 million.

“It’s one of those things — if you’re going to run for governor, you can’t do it at the last minute,” Northam said.

State Sen. Barbara A. Favola (D-Arlington) was among those who wholeheartedly welcomed a Northam bid.

“I think it’s a great idea. He is such a Virginian,” she said. “He sort of gets the thoughtful and methodical way that the Virginia General Assembly likes to conduct its policy. He’s just a very, very good fit for his current job, and he’d be a great governor.”

She was less gung-ho on the notion of a Democratic primary, saying: “I would hope the two gentlemen could work it out.”

“Mark has done a fine job as AG,” Favola continued. “[But] having a primary when you’re running for governor that year can be very tough. It’s tough on voters. It’s tough on donors. It’s just tough all around. And governor’s races are battles. It would be better for the party and better for Virginia . . . if there’s not a primary and we can just pick a front-runner and just go forward.”

On the Republican side, Sen. Mark D. Obenshain (R-Rockingham), who narrowly lost the attorney general’s race to Herring, is widely expected to run for governor.

Other names often mentioned for the GOP nomination include former White House counselor Ed Gillespie, who came close to unseating U.S. Sen. Mark R. Warner (D) last year; Rep. Rob Wittman; and state Sen. Frank W. Wagner (Virginia Beach).

While Northam has begun to spread the word about his plans, he said he had not contacted Herring.

“I just talked to him yesterday about a tie vote,” Northam said, “but I haven’t discussed my next step with the attorney general.”