LOS ANGELES — When Congress passed President Obama’s health care overhaul, a critical compromise provision was that immigrants living in the United States illegally would not be allowed access to publicly subsidized health insurance. Even now, as lawmakers in Washington are debating an overhaul of immigration laws, leaders from both parties are arguing that no federal money should be spent on health care for immigrants on their way to obtaining citizenship.

But not in California, where there are an estimated 2.6 million illegal immigrants. Here, public health officials, elected representatives and advocacy groups are going in the opposite direction, trying to cobble together ways to provide preventive care for such immigrants, who are expected to make up the largest share of the remaining uninsured once the state’s expanded Medicaid program takes full effect.

By many measures, California, with roughly seven million people currently uninsured, more than any other state, is taking the lead in aggressively rolling out changes before January, when most Americans will be required by law to have medical coverage or pay a penalty, the so-called universal mandate. While other states are resisting efforts to expand Medicaid, California has already begun public campaigns to encourage more residents to enroll in subsidized health plans.

But in counties with large immigrant populations like Los Angeles, officials say that not including immigrants in coverage, regardless of their legal status, will only cost local government more in the long run. So they are lobbying state and federal officials to find ways to pay for preventive health care, rather than rely on emergency rooms to care for them.