WASHINGTON — Tom Perriello, the former congressman from Virginia, is making a surprise entry into his state’s governor’s race, disrupting Democrats’ well-laid plans in what promises to be the most-watched election in the country this year.

Mr. Perriello on Wednesday telephoned Lt. Gov. Ralph S. Northam, who had been widely presumed to be the party’s nominee, to tell him he was entering the race, according to three Democrats directly familiar with the conversation.

Mr. Perriello, a State Department official who is close to President Obama, is expected to announce his intention on Thursday to compete in the June 13 Democratic primary. He did not respond to a voice mail and text message. Mr. Northam could not be reached, either.

By entering the race to succeed Gov. Terry McAuliffe, a Democrat who is limited to a single term, Mr. Perriello will hinder Mr. McAuliffe’s effort to avert a contentious primary. Leading Democrats have sought to clear the field for Mr. Northam, hoping to give him a head start in fund-raising and organization, while at least three Republicans vie for the nomination.