Three of Sonoma County's five supervisorial seats are at stake on Tuesday, with the race to replace retiring incumbent Valerie Brown shaping up as a geographic tug of war between Santa Rosa and Sonoma Valley.

Santa Rosa voters could play a decisive role in the contest that pits two Santa Rosa City Council members — Susan Gorin and John Sawyer — against four candidates from the valley.

And Santa Rosa's influence also could shape the outcome of the contest between incumbent Efren Carrillo and former West County supervisor Ernie Carpenter.

In the third race in the heart of the city, incumbent Shirlee Zane is being challenged by Tim Smith of Rohnert Park, whom she defeated four years ago.

Oakmont, the retirement community on Santa Rosa's eastern flank, is a prime battleground in Brown's First District, with no incumbent running for the first time in decades.

Its residents are frequent voters, the type who dominate low-turnout primary elections like Tuesday's, in which about half of registered voters are expected to cast ballots.

"I don't think I can call anything," said Brown, a former state assemblywoman who is stepping down after two terms as a supervisor. "There are lots of wild cards out there."

Slightly more than half of the First District voters reside in Santa Rosa, which raises the possibility of Gorin and Sawyer emerging as the top two vote-getters on Tuesday and battling for the seat in November.

Both are known in the valley, whereas the four valley candidates — Mark Bramfitt, Gina Cuclis, Michael McClure and Joanne Sanders — are relatively unknown to Santa Rosa voters, said Brian Sobel, a Petaluma political consultant.

In any case, no one is expected to get the 50 percent plus one vote needed to sew up a victory in the primary.

In the West County race, the question is whether Carrillo, the county's first Latino supervisor and a west Santa Rosa resident, can win outright on Tuesday against two challengers from the environmental camp, Carpenter of Sebastopol and Veronica Jacobi, a former Santa Rosa councilwoman.

The sprawling Fifth District, which runs from the west side of Santa Rosa to the coast and up to the Mendocino County line, has a strong green streak. Carpenter represented the district from 1980 to 1996.

But Carrillo, who shot from political obscurity to finish first in an eight-candidate primary at age 27 in 2008, is known for wearing out shoes walking door to door.

In the Third District, Zane is trying to fend off a challenge from Smith. The Third District covers central Santa Rosa and Rohnert Park.

You can reach Staff Writer Guy Kovner at 521-5457 or