SAN FRANCISCO -- Ed Lee will announce today that he is running for a full four-year term as San Francisco mayor, vaulting himself to the top of a crowded field of candidates while breaking a promise not to run that sealed his appointment as interim mayor seven months ago.

"I've changed my mind," Lee told The Chronicle in an interview Sunday. "I know it might be hard for people to understand that change ... but my change of mind in seeking this office has everything to do with wanting what's best for this city."

Lee, 59, said he will formally list himself as a candidate in the November election this morning at the city's Department of Elections, marking a dramatic shift for him during a seven-month stint at the pinnacle of city power.

He said he had no designs on running for mayor back in January. That's when the avuncular city administrator with a penchant for goofy jokes had to be talked into taking the job temporarily after then-Mayor Gavin Newsom was elected lieutenant governor.

When the Board of Supervisors formally appointed Lee on Jan. 11 to serve out the remainder of Newsom's term, he became San Francisco's first Asian American mayor and the toast of its Chinese community.

Lee was supposed to be an effective manager divorced from politics. Now, the man who earlier this year repeatedly said he was looking forward to returning to his job as city administrator is preparing to wage a vigorous campaign, saying a new air of cooperation during his term has inspired him to continue as mayor.

Forum tonight

Lee said he'll make his case immediately, participating in a candidates' forum tonight at the Castro Theatre.

His common refrain in his early months as mayor was clear: "I am not a politician." But tonight he'll share the stage with nine other serious contenders who have been in the race for months, including three who backed him in a crucial early vote by the board of supervisors to put him in office.

The idea at the time was to place the city in capable hands without conferring the benefits of incumbency to someone looking to win the office outright. After all, many on the board wanted to run for mayor themselves.

Now Lee supervises dozens of staff who work on city policy, commands the media attention that accompanies the mayor's office and has the advantage of showing voters what he would do in the job.

Some Lee backers had suggested the once-reluctant politician wouldn't need to campaign, only put his name on the ballot and then continue doing the city's business. Lee disagrees.

"I'm behind. Everyone else is ahead of me. I'm jumping right in," Lee said. "I'm not taking anything for granted just because I'm mayor."

Lee insists he's still the same politically averse person who dialed down the rancor at City Hall and opened his door to all comers - a process he described as "talk people to death until we get an agreement."

"Ed Lee is not changing," he said. "I've changed my mind, but I'm still the Ed Lee that they know."

But Lee's ability to work well with the Board of Supervisors, the body that appointed him and approved a change in city law to allow him to return to his old job as administrator, may be in jeopardy - along with his policy agenda.

Some supervisors on the 11-member board have voiced strong support for other mayoral candidates. Two supervisors, President David Chiu and John Avalos, are candidates themselves.

Chiu, in particular, has been in policy lockstep with Lee after breaking with Avalos and other allies on the board's left flank to back Lee as interim mayor. Since then, Chiu has been central to advancing the mayor's agenda - including a tax break for new hires at companies in the city's Mid-Market area designed to keep Twitter in San Francisco. Lee counts it among his major accomplishments.

"Our successes this year have been based on trust, and many San Franciscans share the widespread disappointment that Ed Lee has broken his promise to our city," Chiu said Sunday after learning of Lee's candidacy. "Ultimately, it will be up to the voters to measure the character and vision of those who want to lead San Francisco for the next four years."

A 'no' vote

Former Supervisor Sophie Maxwell, who with Chiu was among the six supervisors who voted to appoint Lee as interim mayor in a crucial early round of voting, has said Lee will no longer be getting her vote if he enters the race.

"Would Ed Lee be one of my three people?" Maxwell said late last month, referring to the city's ranked-choice voting system. "No, because he said he would not run."

Lee said he will now try to sit down with those supervisors and explain his rationale.

"I understand if they feel anger and frustration that I made a promise I wouldn't run," Lee said. "I will sit down and explain to them why I changed my mind."

The switch, he said, was driven by a belief that San Francisco politics had changed dramatically from a blood sport to a practical, consensus-driven system in his seven months on the job.

He cited his ability to craft a balanced budget without deep cuts or major fights and to negotiate a public-sector pension reform proposal for the November ballot supported by both budget-hawk Supervisor Sean Elsbernd and organized labor.

"This city has changed. The way we run government is changing," Lee said. "I want to continue that change. This new tone ... it changed my mind."

Lee's announcement comes as the city's Ethics Commission is to hold a hearing today on campaign finance activity by Progress for All, the group behind the "Run, Ed, Run" campaign. It was funded overwhelmingly by figures with city contracts or links to former Mayor Willie Brown or Chinese Chamber of Commerce consultant Rose Pak, a close friend of Lee's. Lee maintains he had no involvement with the group.

A critical moment

Lee said a critical moment in his decision to run came when he found himself waiting outside the White House on July 25. The San Francisco Giants had a reception with President Obama, but Lee's name wasn't on the guest list.

He ran into Sen. Dianne Feinstein, D-Calif., the last person to fill a vacancy in the San Francisco mayor's office. She was appointed in 1978 to serve the remainder of assassinated Mayor George Moscone's term, then was elected the next year to the first of her two terms.

"We had a heart-to-heart talk, and that affected me," Lee said. "It really helped me see the change that had taken place" in the city.

Inside

Reaction: Supervisors criticize Lee for breaking pledge. A10

Matier & Ross: Lee is popular, but falls short of a majority, poll finds. C1