HURON, Calif. — When Chuck Herrin, who runs a large farm labor contracting company, looks out at the hundreds of workers he hires each year to tend to the countless rows of asparagus, grapes, tomatoes, peaches and plums, he often seethes in frustration.

It is not that he has any trouble with the laborers. It is that he, like many others in agriculture here, is increasingly fed up with immigration laws that he says prevent him from fielding a steady, reliable work force.

“What we have going on now is a farce — a waste of time and money,” said Mr. Herrin, a lifelong Republican who grew up in central California, adding that the country should be considering ways to bring workers in, not keep them out. “We need these people to get our food to market.”

California is home to an estimated 2.5 million illegal immigrants, more than in any other state. Perhaps nowhere else captures the contradictions and complications of immigration policy better than California’s Central Valley, where nearly all farmworkers are immigrants, roughly half of them living here illegally, according to estimates from agricultural economists at the University of California, Davis.

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Chuck Herrin, left, with an employee, Alvaro Martinez, says he struggles to fill his work crews. The work force is aging, Mr. Herrin said, as potential immigrants are deterred from coming to the region. CreditMatt Black for The New York Times

That reality is shaping the views of agriculture business owners here, like Mr. Herrin, who cannot recall ever voting for a Democrat. In dozens of interviews, farmers and owners of related businesses said that even the current system of tacitly using illegal labor was failing to sustain them. A work force that arrived in the 1990s is aging out of heavy labor, Americans do not want the jobs, and tightened security at the border is discouraging new immigrants from arriving, they say, leaving them to struggle amid the paralysis on immigration policy. No other region may be as eager to keep immigration legislation alive.

The tension is so high that the powerful Western Growers Association, a group based in Irvine, Calif., that represents hundreds of farmers in California and Arizona, says many of its members may withhold contributions from Republicans in congressional races because of the party’s stance against a comprehensive immigration overhaul.

Mr. Herrin says he is constantly shifting his work force during harvest, and can often provide crews only half the size that farmers request. Like other employers interviewed, he acknowledged that he almost certainly had illegal immigrants in his work force. Would-be workers provide a Social Security number or a document purporting they are eligible to work; employers accept the documentation even if they doubt its veracity because they want to bring in their crops.

“We have no choice,” he said. “We are not getting people who are coming out of the towns and cities to come out and work on the farms.” Potential workers, he said, are “scared to come, scared of Border Patrol and deportations and drug lords. They can’t afford to risk all these things.”

Roughly a third of Mr. Herrin’s workers are older than 50, a much higher proportion than even five years ago. He said they had earned the right to stay here. “If we keep them here and not do anything for them once they get old, that’s really extortion,” he said.

The region has relied on new arrivals to pick crops since the time of the Dust Bowl. For more than two decades after World War II, growers here depended on braceros, Mexican workers sent temporarily to the United States to work in agriculture. Today, many fieldworkers are indigenous people from southern Mexico who speak Mixtec and know little English or Spanish.

In recent years, farm owners have grown increasingly fearful of labor shortages. Last year, the diminished supply of workers led average farm wages in the region to increase by roughly $1 an hour, according to researchers at U.C. Davis who have tracked wages for years. Now, farm owners are pressing to make it easier for would-be immigrants to obtain agricultural visas, which they say would create a more reliable labor supply.

A report released this month by the Partnership for a New American Economy and the Agriculture Coalition for Immigration Reform, two business-oriented groups that are lobbying Congress, said foreign-grown produce consumed in the United States had increased by nearly 80 percent since the late 1990s. The report argues that the labor shortages make it impossible for American farmers to increase production and compete effectively with foreign importers. While the amount of fresh produce consumed by Americans has increased, domestic production has not kept pace, and the report attributes a $1.4 billion annual loss in farm income to the lack of labor.

So even amid a record drought threatening to wipe out crops here, growers routinely talk of immigration as a top concern, saying they are losing some of their most valuable workers because of deportations or threats of being sent away. Kevin Andrew, the chief operating officer for Jakov P. Dulcich and Sons, which grows grapes and other produce in the region, remembers what happened to one of his workers who was simultaneously up for a promotion and citizenship a couple of years ago.

“Just as he goes to his final interview, they found some document where his two last names were reversed and they came after him for attempting to defraud the government,” Mr. Andrew said. “This is a guy who owned two or three homes, had stellar letters written for him by supervisors, and they’re looking for a reason to count him out.

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An orange grove in California’s Central Valley, where farmers cite immigration as a top concern. CreditMatt Black for The New York Times

“He came to me afterward and was crushed, just sobbing like a baby. All of a sudden he can’t be a supervisor because he’s wanted by the government. He was supposedly living the American dream, and they just took everything away in an instant.”

Mr. Andrew saw the man several months later, working at a job that paid less than what he had been earning for years.

Huron is part of an unusual congressional district: It is more than two-thirds Latino and is represented by a Republican, David Valadao. No other district represented by a Republican has more illegal immigrants. Mr. Valadao and Representative Jeff Denham, who represents a northern stretch of the agricultural valley, are two of the three Republicans who support a Democratic-sponsored bill that would grant a legal path to citizenship for millions of undocumented immigrants.

“There are people who have been employed for many years, if not decades, and are now turning to their employers saying, ‘Look, I am undocumented,’ ” Mr. Denham said in an interview. “These are not just seasonal workers. These are people who have almost become part of the same family. It’s a problem that has grown so big and so multigenerational, we can no longer ignore it.”

After decades of immigration, the region has become home to many of the children of Mexican laborers. Mr. Denham, for example, is married to the daughter of a former bracero from Mexico who became a citizen decades after he arrived in the Central Valley.

Industry groups are among the most important forces pressing Congress for an immigration overhaul. Tom Nassif, the president of the Western Growers Association, has shuttled to Washington to press members of Congress, especially Republicans, to get a bill passed this year. Mr. Nassif, an ambassador to Morocco under President Ronald Reagan, has long called for easing entry at the Mexican border to make it easier for growers to find labor.

“We’ve had secure borders with Mexico for the last decade; we don’t have that argument at this point,” Mr. Nassif said. “Now we want people to see the real damage of not doing anything, which is a declining work force, and it means losing production to foreign countries.”

After the 2012 presidential election, as Republicans spoke enthusiastically about the need to court Latinos, Mr. Nassif was optimistic that immigration would become a top priority. But exasperation has replaced his confidence in recent months, and he said his group could withhold hundreds of thousands of dollars in congressional races in which it has usually supported Republicans.

“I can tell you if the Republicans don’t put something forward on immigration, there is going to be a very loud hue and cry from us in agriculture,” Mr. Nassif said. “We are a tremendously important part of the party, and they should not want to lose us.”

Joe Del Bosque grew up in the San Joaquin Valley after his parents came to California as children during the Mexican Revolution in the early part of the last century. A generation ago, he said, growers often pretended to have no idea that people working for them were not authorized to be in the United States. Now, there is a nearly universal recognition that the industry relies on immigrants who cross the border illegally.

Like other growers in the area, he said he felt politically isolated. “The employers are more frustrated than the actual immigrants,” said Mr. Del Bosque, who grows cantaloupes, almonds and asparagus near Los Banos, north of Fresno.

“I thought it would have been much more contentious for them, but they are not so demanding,” he said. “It’s not a revolution for them — it’s more for us.”