RICHMOND — Former Virginia congressman Tom Perriello told senior Democrats on Wednesday that he intends to run for governor, a surprise move that would belatedly thrust Gov. Terry McAuliffe’s hand-picked successor into a primary battle.

Until now, Lt. Gov. Ralph Northam has been the only Democrat seeking to succeed McAuliffe, who is barred by the state constitution from seeking back-to-back terms.

In a round of phone calls Wednesday, the one-term congressman told leading Democrats that he plans to jump into the race, according to four Democrats familiar with the conversations. Perriello called Northam directly to tell him that he was running and said he would publicly announce his plans Thursday, according to two of the Democrats, all of whom spoke on condition of anonymity to discuss private conversations.

Perriello did not respond to a message seeking comment.

But his potential candidacy was confirmed by Emily Bolton, communications director for the state Democratic Party. She said an announcement would come Thursday.

Former Virginia congressman Tom Perriello. (Melina Mara/The Washington Post)

Some leading Democrats were upset that Perriello would — just five months before the June 13 primary — threaten to disrupt what had been an unusually unified Democratic front.

While Republicans have been tearing one another apart in crowded primaries for governor and lieutenant governor, Democrats have been lined up behind Northam for more than a year — ever since Attorney General Mark R. Herring (D) decided in September 2015 to seek reelection instead of the governorship.

Some senior Democrats hoped to persuade Perriello, who has worked overseas for the State Department under President Obama, to stay out of the contest.

“He’s got a better chance of flapping his wings and flying to the moon than getting within 50 points of that nomination,” said state Senate Minority Leader Richard L. Saslaw (D-Fairfax), who had not heard directly from Perriello. “I’ve heard of some harebrained things, but this takes the cake.”

Perriello rode Obama’s coattails in 2008 to narrowly defeat a longtime GOP incumbent in a red-leaning central Virginia congressional district. He lost the seat two years later.

His potential entry into the gubernatorial race was first reported by the New York Times.

Northam, a widely liked but low-key former state senator, is a native of the Eastern Shore, a graduate of Virginia Military Institute, a Gulf War veteran and pediatric neurologist. His campaign issued a statement saying that he still enjoys the support of the state’s leading Democrats.

“Ralph Northam is proud to have the support of Governor McAuliffe and Democrats across Virginia, including all of our statewide elected officials, and will continue to fight back against attempts by Republicans to discriminate against the LGBT community and limit women’s access to reproductive health care,” Brad Komar, Northam’s campaign manager, said in an email.

Brian Coy, a spokesman for McAuliffe, said Northam still has the governor’s backing. “He continues to believe that Lieutenant Governor Northam is the best candidate for governor,” Coy said.

Amy Dudley, a spokeswoman for Sen. Tim Kaine (D-Va.), confirmed that the senator still supports Northam.

But one Democratic elected official who supports Northam said Perriello’s entry into the race could be a positive development.

“I think competition usually is a good thing, as long as they don’t tear each other down and use up too many resources,” said the official, who spoke on condition of anonymity to talk freely about the race.

“There are many voices within our party,” he said.

Gregory S. Schneider contributed to this report.