An immense stretch of ecologically valuable woodlands and open space covering four Bay Area counties has been targeted by Gov. Gavin Newsom and state legislators to become the first major property added to the California state park system since the 1940s.

The governor’s $222 billion budget includes $20 million to acquire potential parkland. During his news conference on the budget Friday, Newsom refused to identify the site, warning that it could boost the cost. But legislators confirmed Saturday that the property in question was the N3 Ranch, an 80-square-mile swath of land an hour’s drive from San Francisco.

The ranch, on sale for the first time in 85 years, includes canyons, woodlands, grasslands and meadows in Santa Clara, Alameda, San Joaquin and Stanislaus counties. The asking price is $72 million.

“This is an extraordinary opportunity to acquire this pristine open space 50 times larger than Golden Gate Park and a few bus stops away from millions of Bay Area residents,” said state Sen. Steve Glazer, D-Orinda, one of 15 Bay Area legislators who signed a letter Jan. 6 urging Newsom to allocate state funds to acquire the ranch.

“It’s pristine wildlands, important Bay Area watershed and it could provide extraordinary passive recreation opportunities on trails and hills,” he said. “This is not something in the foothills of the Sierras. This is in our backyard.”

The idea is to combine the state money with about $30 million raised by the Nature Conservancy, Trust for Public Land and other potential contributors.

Todd Renfrew, the broker and principal owner of California Outdoor Properties of Vacaville, told The Chronicle that he has received interest from local and international buyers, including owners of neighboring ranches.

Some potential buyers have inquired about possibly subdividing the property, but Renfrew said the owners want to keep the property whole.

The land, which includes 200 miles of dirt roads for hiking, running and biking, has been owned by the Vickers and Naftzger families since the early 1930s. It was put up for sale in July by two surviving sisters who live in Southern California and rarely visit the ranch anymore.

The property, which is only 15 minutes from downtown Livermore, includes a four-bedroom home, barns, sheds, 14 hunting camps with cabins, skinning sheds and grazing land for up to 3,200 cows.

“We’re impressed by the stewardship of the N3 Ranch for the past eight decades and its unique conservation values, including the size and scope of the property, its sharp-cragged backcountry terrain, and its habitat for species of interest,” said Mike Sweeney, executive director of the California branch of the Nature Conservancy. “We’d like to see N3 protected.”

The N3 would be difficult to develop because it is remote and rugged, while building on it would require major changes in zoning. The property also is covered by the 1965 Williamson Act, which allows owners to reduce their annual property taxes in exchange for preserving the land as farmland and open space.

The potential property tax hit and the cost of new infrastructure like roads, sewer, electricity and water is why the state and its potential partners believe they can acquire the land for less than the asking price.

There is, however, real value in how much unspoiled habitat is on the land, according to everyone involved. The 50,500-acre property stretches from Tracy to Del Valle Reservoir and from Calaveras and San Antonio reservoirs to both the Sunol and Ohlone regional wilderness areas. The Alameda Creek watershed, which supplies water to San Francisco, runs through the property.

The land includes 4,089-foot Eylar Mountain, skirts the east side of Mission Peak and runs up toward Mount Diablo State Park and Henry Coe State Park. It is habitat for mountain lions, coyote, bobcats, fox, elk, deer and migrating birds.

And it is huge, covering 19,935 acres in Santa Clara, 16,880 acres in Alameda, 4,590 acres in Stanislaus, and 9,095 acres in San Joaquin County. That’s bigger than San Francisco and more than twice the size of nearby Mount Diablo State Park.

“This is a once-in-a-lifetime opportunity,” said Democratic state Sen. Jerry Hill, who represents San Mateo and Santa Clara counties and also signed the letter. “This would protect acres of ranchland and wilderness by creating a new state park. It would pay dividends for generations to come in an area that needs recreational opportunities.”

California, which created its state park system in 1864, has about 280 parks, more than any other state. They cover 1.5 million acres, including 280 miles of coastline and 625 miles of lake and riverfront. Only Alaska has more land, 3.2 million acres, devoted to state parks.

The park system has recently been plagued by budget shortfalls, requiring closures a few years ago. A park the size of N3 hasn’t been added to the system in more than 70 years.

“If we don’t acquire this we lose the opportunity,” Glazer said. “This would be a new state park of extraordinary size and importance.”

Peter Fimrite is a San Francisco Chronicle staff writer. Email: pfimrite@sfchronicle.com Twitter: @pfimrite