PATTERSON, Calif. — Farmers across the country are rallying to fight a Republican-sponsored bill that would force them and all other employers to verify the legal immigration status of their workers, a move some say could imperil not only future harvests but also the agricultural community’s traditional support for conservative candidates.

The bill was proposed by Representative Lamar Smith, a Texas Republican who is the chairman of the House Judiciary Committee. It would require farmers — who have long relied on a labor force of immigrants, a majority here without legal documents — to check all new hires through E-Verify, a federal database run by the Department of Homeland Security devised to ferret out illegal immigrants.

Farm laborers, required like other workers to show that they are authorized to take jobs in the United States, often present Social Security numbers and some form of picture ID. Employers, many of them labor contractors providing crews to farms, have not been required to check the information and are discouraged by antidiscrimination laws from looking at it too closely. But it is an open secret that many farmworkers’ documents are false.

Supporters of E-Verify, an electronic system that is currently mandatory for most federal contractors but voluntary for other employers, argue that it would eliminate any doubt about workers’ legal status. But farmers say it could cripple a $390 billion industry that relies on hundreds of thousands of willing, low-wage immigrant workers to pick, sort and package everything from avocados to zucchini.