MERCED, Calif. — Jeff Marchini and others in the Central Valley here bet their farms on the election of Donald J. Trump. His message of reducing regulations and taxes appealed to this Republican stronghold, one of Mr. Trump’s strongest bases of support in the state.

As for his promises about cracking down on illegal immigrants, many assumed Mr. Trump’s pledges were mostly just talk. But two weeks into his administration, Mr. Trump has signed executive orders that have upended the country’s immigration laws. Now farmers here are deeply alarmed about what the new policies could mean for their workers, most of whom are unauthorized, and the businesses that depend on them.

“Everything’s coming so quickly,” Mr. Marchini said. “We’re not loading people into buses or deporting them, that’s not happening yet.” As he looked out over a crew of workers bent over as they rifled through muddy leaves to find purple heads of radicchio, he said that as a businessman, Mr. Trump would know that farmers had invested millions of dollars into produce that is growing right now, and that not being able to pick and sell those crops would represent huge losses for the state economy. “I’m confident that he can grasp the magnitude and the anxiety of what’s happening now.”

Mr. Trump’s immigration policies could transform California’s Central Valley, a stretch of lowlands that extends from Sacramento to Bakersfield. Approximately 70 percent of all farmworkers here are living in the United States illegally, according to researchers at University of California, Davis. The impact could reverberate throughout the valley’s precarious economy, where agriculture is by far the largest industry. With 6.5 million people living in the valley, the fields in this state bring in $35 billion a year and provide more of the nation’s food than any other state.

The consequences of a smaller immigrant work force would ripple not just through the orchards and dairies, but also to locally owned businesses, restaurants, schools and even seemingly unrelated industries, like the insurance market.

Many here feel vindicated by the election, and signs declaring “Vote to make America great again” still dot the highways. But in conversations with nearly a dozen farmers, most of whom voted for Mr. Trump, each acknowledged that they relied on workers who provided false documents. And if the administration were to weed out illegal workers, farmers say their businesses would be crippled. Even Republican lawmakers from the region have supported plans that would give farmworkers a path to citizenship.