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.