FREMONT – The last auto plant in California is threatened with closure this fall, which would silence an economic engine responsible for an estimated 20,000 jobs throughout the state.

Toyota Motor Corp. is considering shutting the New United Motor Manufacturing Inc. plant, its history-making joint venture with General Motors Corp., in a move that would worsen California's recession and strike deep into the Central Valley.

Though it's a mainstay of the East Bay, NUMMI's influence is widespread. Nearly half its 4,500 workers live in the Stockton-Modesto area, union leaders say. Hundreds of additional workers make parts for NUMMI at factories up and down the Valley, where unemployment is already well into the double digits.

"Auto assembly plants have long supply lines," said Jeff Michael, an economist at Stockton's University of the Pacific. "There's a number of enterprises that basically have one customer, and it's that plant.

While economists often talk about "ripple effects," closing NUMMI "would have the biggest ripple effect of anything I can think of," Michael said.

Until now, NUMMI has long been a symbol of the rebirth of the U.S. auto industry, a successful marriage of American and Japanese manufacturing plucked from the wreckage of a dysfunctional GM plant that closed in 1982.

But the collapse of the auto industry in the past year has brought NUMMI (rhymes with "roomy") to its knees. Production has fallen at least 25 percent and GM, following its bankruptcy reorganization, is pulling out of the joint venture. Production of the only GM vehicle made here, the Pontiac Vibe, is winding down.

GM's departure put Toyota on the spot. With its worldwide sales down 38 percent from last year, Toyota said in late July that it is considering dissolving the joint venture, putting the plant in jeopardy.

Shutdown wouldn't come for a while. Plant executives told union leaders that the government's "cash for clunkers" program had produced enough orders for Toyota Corollas and Tacoma trucks to keep the plant going through October.

Beyond that, the future is cloudy. Toyota Chief Executive Akio Toyoda, a former NUMMI manager, said during a speech in Michigan last week that Toyota expects to decide soon whether to keep the plant open.

Union leader Sergio Santos believes the plant can be rescued but said his membership is rattled. "It's got a lot of people scared," said Santos, president of Local 2244 of the United Auto Workers and a 19-year NUMMI veteran.

California officials have gone into overdrive. Bills have been introduced in the Assembly and Senate to exempt the plant from paying sales and use taxes on new plant and equipment.

Gov. Arnold Schwarzenegger and local officials, in a recent letter to Toyota's chief executive in Tokyo, offered a trunkful of additional incentives. They include up to $29 million in interest-free loans from the city and Alameda County to retool the factory, said Fremont Mayor Bob Wasserman.

"We think there's definitely a fighting chance to save the plant," said David Crane, the governor's special adviser for jobs and economic growth. California represents Toyota's largest U.S. market, making it advantageous for the company to keep the plant open, Crane added.

Inside Toyota's executive suite, though, NUMMI's fate is more complicated.

The company takes pride in never having closed a plant, and wouldn't be contemplating a shutdown at NUMMI if GM hadn't pulled out, said Jeffrey Liker, a University of Michigan industrial engineering professor who's written books on Toyota.

But experts say Toyota has far too much manufacturing capacity, including a new plant in Mississippi that's sitting idle. Liker said NUMMI lost money last year and is likely to remain unprofitable in the foreseeable future.

"They feel an obligation to California and that community," Liker said. "They definitely feel that sense of obligation, but at what cost?"

A key issue might be labor expense. The UAW's contract expires Thursday, and it's widely expected that Toyota will seek concessions. Workers currently make about $28 an hour in salary, Santos said. Wages at newer, nonunion plants in the Southeast average $15 to $20 an hour, according to Automotive News.

Call The Bee's Dale Kasler, (916) 321-1066. Read his blog on the economy, Home Front, at www.sacbee.com/blogs.