Santa Rosa City Councilwoman Susan Gorin confirmed Friday that she is running for the Sonoma County Board of Supervisors seat to be vacated by Valerie Brown, a move that will pit her against a rival city politician and play up a long-standing geopolitical divide in the county, observers said.

Gorin, 59, announced her decision Thursday night in Petaluma at a quarterly gathering of Sonoma County mayors and city council members.

She had shared her plans with some in attendance and felt it was the right time to clear the air about her intentions, she said.

"I didn't want to duck the question. I felt like I really needed to let people know that I'm moving forward," she said Friday.

The decision carries immediate personal implications for Gorin. By mid-February she will have to move into the county's 1st District from her Fountaingrove home, which rests just outside the current boundaries.

Other candidates have said the move will be an issue in the campaign. But Gorin brushed off those suggestions, saying the challenges facing county government — she discussed rising retirement costs and cuts to public services — and her experience in public leadership posts would be more important to voters.

She said county election officials have assured her that her current residence does not prevent her from campaigning or raising funds.

"People are looking at my experience and my ability to represent the Sonoma Valley," she said. "I wouldn't be making this decision if I hadn't talked with people in the district."

Gorin has served almost five years on the Santa Rosa council, including two years as mayor from 2008 to 2010. She was re-elected last year as the top vote-getter in the race but was ousted from the majority in a shakeup that her critics cast as a rebuke of her leadership.

Her current council term expires in 2014.

Previously, Gorin served as a trustee on the Santa Rosa School Board and as a member of the city's Planning Commission and Board of Public Utilities.

Three other have announced their candidacies for the 1st District seat, which takes in all of Sonoma Valley and extends into east Santa Rosa, including Oakmont, Bennett Valley and Rincon Valley. By next year, because of redistricting, it will include more of Fountaingrove, but not enough to include Gorin's home off Bent Tree Place.

Among the challengers is John Sawyer, a fellow Santa Rosa council member whose newly-elected allies last year helped form a new majority that excluded Gorin.

The rivals draw support from the county's two main dueling political camps. Gorin has been backed by the more liberal spectrum, including environmental groups and some labor groups, while Sawyer's supporters come more from business and development interests.

The politicians are civil together — Gorin said they shared a hug after her announcement Thursday — but the simmering feud and policy differences between their two sides likely will play a key role in the contest, political observers said.

That would be a change for a district traditionally based in the Sonoma Valley, a region apart from the Highway 101 corridor that includes Santa Rosa, said David McCuan, the Sonoma State University political scientist.

"It immediately puts Santa Rosa politics in the center of the stage," he said, adding that 1st District voters could see that as "meddling" and turn toward the other two candidates, Mark Bramfitt and Gina Cuclis, both of whom come from Sonoma Valley.

Cuclis, who lives in Boyes Hot Springs, said the distinction would help set her apart from Gorin, who shares many of the same environmental priorities.

"I'm already clearly able to stake out my differences because I am a resident of the unincorporated area," said Cuclis, a communications consultant and past Sonoma planning commissioner.

Gorin on Friday touted her service on regional agencies overseeing water supply, air quality, land-use and transportation planning.

Along with environmental protection, she said that improving the county's economic vitality through development would be a priority as as a supervisor.

"That balance, that sensitive attention to constituents' needs, is going to be crucial for the election and the future of Sonoma Valley," Gorin said.