Douglas M. Duncan, who abruptly dropped his bid for governor six years ago amid a diagnosis of depression, has told supporters that he is running for his former job as Montgomery County’s executive.

The decision, which Duncan has been mulling for months, resurrects the political career of one of the Washington region’s most dynamic public figures, a gregarious and at times tempestuous politician whose forceful leadership led to the revitalization of downtown Silver Spring.

It also immediately reshapes the 2014 Democratic primary race, in which several high-profile county officials have expressed an interest. County Executive Ike Leggett (D), who succeeded Duncan, has also indicated that he may run again, despite earlier pledges to the contrary.

Sometimes called the “Mayor of Montgomery,” Duncan, 57, announced his decision before a crowd of 50 to 100 business leaders at a Gaithersburg hotel Tuesday. There, his pollster Harrison Hickman, who has worked for the Al Gore and John Edwards presidential campaigns, discussed a poll this year that showed Duncan had strong name recognition and favorability ratings.

“Make no mistake,” Duncan, 57, said, according to supporters who attended the meeting. “I am running for county executive.”

Former Montgomery County executive Doug Duncan with wife, Barbara, listen to speakers at an Oct. 7 political rally for Maryland 6th District Congressional candidate John Delaney. Duncan has announced he will seek the county executive job again. (Katherine Frey/The Washington Post)

His entrance into the race marks a dramatic return for Duncan, the ­county’s longest-serving executive, who soon disappeared from public view after dropping out of the 2006 Democratic primary against then-Baltimore Mayor Martin O’Malley.

In a hastily arranged news conference to announce his decision not to pursue the governorship, he said it had become clear to him that the depression he was feeling “was more than the usual wear and tear of the campaign trail.”

Shortly afterward, he said in a speech commemorating the 50th anniversary of the Mental Health Association of Montgomery County that “it didn’t take any courage to stand up and say, ‘Here’s my problem.’ The courage it took for me was to admit I have a problem, to admit that I needed help and to seek that help.”

At the time, he said he was getting treatment and taking medication. His family has a history of mental health issues, campaign aides said at the time.

In an interview last month, Duncan said his mental health is much better: “I’m feeling good.”

One of his supporters, who was not authorized to speak publicly about Duncan’s run, said that he’s prepared for the issue to come up in the campaign and that it is “something that he has to address head on, and he’s more than willing to do it.”

Duncan did not respond to requests for comment Wednesday.

As county executive, the 6-foot-4 Duncan left a large impression on the county, masterminding the revitalization of Silver Spring and the $96 million concert hall at Strathmore. He was widely praised for his calming demeanor during the 2002 sniper crisis. He ran on a developer-friendly platform, saying that the county suffered from “paralysis by analysis.”

Despite his many successes, he also was at times a controversial figure, upsetting advocates who wanted the county to grow more slowly. And shortly before he dropped out of the governor’s race, The Washington Post reported that in 1999 he had accepted $20,000 in campaign money from businesses linked to convicted former lobbyist Jack Abramoff weeks before he authorized the lease of a shuttered public school to a Jewish organization Abramoff supported. Duncan later returned the money.

During his tenure, the county budget more than doubled, to nearly $4 billion, and the population surged by about 20 percent. He was a constant presence in Annapolis, fighting for state dollars and earning the nickname “Delegate Duncan.”

After Duncan ended his gubernatorial bid in 2006, he underwent hip-replacement surgery. He did a few public appearances during his six-month recovery. In a 2009 radio interview, he said it had been hard for him to wade back into public view: “It was very difficult for me to be in public and for me to say things in public without sort of thinking that these people think I’m a fraud and that they’re just laughing at me.”

In 2007, he became vice president for administrative affairs for the University of Maryland at College Park. During his tenure, he spearheaded a large development deal to build a town center and expand university facilities on what is known as East Campus, near the College Park Metro station. But he resigned in 2008 after a dispute with O’Malley. Duncan asserted that O’Malley did not want him participating in a forum alongside the governor’s political foe and predecessor, Robert L. Ehrlich Jr. (R).

Duncan then co-founded a government advisory firm called CivicUS, which soon dissolved. He later joined a statewide transportation advocacy group and went into consulting on his own. Last year, he was hired by Foulger-Pratt, a major developer in the county, to observe debates over a proposed youth curfew. Duncan quietly sat in on county hearings, listening while reading on his iPad. Duncan also worked with a Gaithersburg software company and recently started work with Lerner Enterprises to get state approval for a hospital on one of the company’s properties in Prince George’s County.

As he began consulting, Duncan considered a run for Congress, meeting with key former advisers. But by the time the Montgomery County Council indefinitely postponed the curfew vote in December, he had decided not to run.

“In the end I decided I’m an executive, not a legislator,” Duncan said in November. Within weeks, he endorsed businessman John Delaney, recording a campaign radio ad for him and actively campaigning for him. Delaney won the 6th District seat.

Whispers of Duncan’s return to politics have been circulating for months. He could be entering a crowded Democratic primary for county executive. Council member Phil Andrews (Gaithersburg-Rockville) has said he is running, and George Leventhal (At Large) has expressed strong interest. Others, including Nancy Floreen (At Large), Valerie Ervin (Eastern County), Marc Elrich (At Large), state Del. Benjamin F. Kramer and the county’s economic development director, Steve Silverman, are also considering it.

Leggett, who repeatedly has said he wanted to leave after this term, is weighing another run. In an interview Wednesday, he said he plans to meet with supporters next year to announce his decision.

Duncan is making clear to others that he thinks the county is going in the wrong direction. At a breakfast last month with business leaders in Bethesda, he said he thinks state officials in Annapolis are paying less attention to the county than in previous years.

He added that he thinks the county is “slipping” against Northern Virginia when it comes to attracting businesses.

“I’m concerned that Montgomery County is getting a much bigger reputation that it is being anti-business,” he told the group.

In an e-mail to supporters, he wrote: “I am not seeking to return to the County Executive’s office simply because it is winnable. I am returning because I have so much energy for the job and know that Montgomery County is ready to again see real progress.”

Ann E. Marimow and Miranda S. Spivack contributed to this report.