With a population of just 55,000 scattered among its hills, San Benito County seems an unlikely threat to California’s oil industry.

But come Tuesday, voters there will decide whether to ban fracking, acidizing and other “high-intensity” forms of oil extraction within the county’s borders. And the industry isn’t taking the challenge lightly.

San Benito is one of three California counties with fracking bans on this week’s ballot. And together, oil companies including Chevron Corp. and Occidental Petroleum have spent $7.7 million to defeat them. That’s more money than California’s Republican gubernatorial candidate, Neel Kashkari, has raised during his entire campaign.

Only one of the counties, Santa Barbara, is a major oil producer. San Benito has just 26 wells, and none has been fracked. The third county, Mendocino, has no active oil wells, according to state records.

But ban proponents consider hydraulic fracturing — which uses pressurized water, sand and chemicals to crack open oil-bearing rocks — a danger to California’s ground water supplies, particularly at a time when three years of historic drought have drained aquifers. Better to stop it now, they say.

“To me, the water is the story,” said Tom Shepherd, an organic farmer in Santa Barbara County’s Santa Ynez Valley. “Here we are, in the midst of a drought, and you’re not concerned about your water and fracking? The aquifers have been drawn down. The rain we’ve had has been absorbed by the ground. The creeks don’t run.”

Oil companies consider fracking safe, saying they’ve used it in California for decades without a single documented case of groundwater contamination. They consider the proposed bans little more than an attempt to strangle oil production in the state.

“No one, to my knowledge, has fracked a well in San Benito County, and to my knowledge, no one is proposing to do that,” said Steve Coombs, whose small oil company, Patriot Resources, has a dozen wells in the county. “So why are we having an initiative about banning fracking? At the end of the day, it’s anti-fossil fuels.”

Fracking opponents have pushed repeatedly for a statewide ban or moratorium, only to see the idea die in Sacramento. Last year, Gov. Jerry Brown threw his weight behind legislation that let fracking continue while creating new regulations for the practice and launching a study of its potential dangers.

Frustrated, environmentalists shifted their focus to local politics. Santa Cruz County adopted a moratorium last year. The Los Angeles City Council followed suit in February.

The vast majority of fracking reported in California to date has taken place in one county — Kern, the heart of the state’s oil industry. No one expects a ban there anytime soon. But other counties may follow if San Benito, Santa Barbara and Mendocino voters block fracking. That, ban supporters say, explains the industry’s heavy spending.

“They might think that, as goes Santa Barbara County, so goes the state,” said Rebecca Claassen, a chiropractor and co-founder of Santa Barbara County Water Guardians.

Chevron, based in San Ramon, has donated $2.6 million to defeat the ballot measures, according to state records. Aera Energy has given $2.1 million; Occidental Petroleum, $2 million.

Backers of the Santa Barbara ban report raising $400,000. Their counterparts in San Benito have raised $120,000.

The ballot fight comes as oil companies try to develop the Monterey Shale, a vast oil-laden rock formation beneath the southern San Joaquin Valley. Although the federal government this year slashed its estimate of the amount of oil that can be squeezed from the shale using current technology, drillers continue probing the formation, saying it could one day yield an economic bonanza for the state.

The idea of an oil boom, however, troubles many rural residents. Fracking for oil and natural gas has transformed swaths of North Dakota, Pennsylvania and Texas, filling them with drill rigs, truck traffic and air pollution.

In San Benito County, a company called Citadel Exploration has been probing for oil on a 688-acre site that the company says holds enormous promise. Developing the field, near Pinnacles National Park, would likely require “several hundred wells,” Citadel announced in August.

Given the scale of California’s oil industry, San Benito residents pushing for a ban doubt the ability of state regulators to protect them. The state government doesn’t have the resources to make sure every well is correctly designed and drilled, said Andy Hsia-Coron, a retired teacher who helped launch the ban proposal.

“I don’t think they’re up for policing this process across the state,” he said. “From early on, I thought it was going to require counties to defend themselves.”

The proposed bans in San Benito and Santa Barbara counties, both written by the same San Francisco law firm, don’t just target fracking. They would also block acidizing, which uses acid to create tiny channels in subterranean rocks. And they would prohibit “steam flooding,” in which steam is pumped underground to heat thick, heavy oil and make it flow more easily.