LOS ANGELES » With an estimated 15 percent of the country’s uninsured population, California is crucial to the success of President Barack Obama’s health care overhaul. Here, that success cannot come without enrolling Latinos, who make up more than half of the state’s uninsured.

But so far, enrollment of Latinos has fallen strikingly below the hopes of the law’s proponents, accounting for 20 percent or less of those who signed up on the state-run health insurance exchange by the end of December. Now, state officials are rushing to expand marketing efforts and hire additional Spanish-language staff, hoping to sharply increase that number by March 31, when open enrollment in the new insurance plans ends.

"For the Affordable Care Act to work, everyone needs to get in. And if we’re relatively unsuccessful with a major segment of who should be getting insured, that would undoubtedly mean that we’re unsuccessful with the entire enterprise," said Peter Lee, the executive director of Covered California, the state’s online health care marketplace. "We’re absolutely doubling down."

Enrollment problems are playing out in other states with large Hispanic populations as well, including Arizona and Texas, counselors who work with immigrants said. But California stands out as the most troubling case, proponents of the law say, because it not only has the most Latinos of any state but also has worked so hard to enroll them.

For months leading up to the rollout of the Affordable Care Act, state officials conducted focus groups and drew up elaborate marketing plans, with groups spending more than $100 million to publicize the new law, including extensive Spanish-language campaigns. And while many Republicans and conservative state leaders have criticized the new health care system, polls show that Latinos embrace the change.

Many critics simply blame language for the slow start among Latinos — Covered California did not offer applications in Spanish until the end of December, and the Spanish language website was littered with translation errors for weeks after its start. Others say there are not enough counselors to provide the kind of personal attention many people who have never had health insurance need.

But fears of deportation may be a major factor, too. Health care clinics and community groups who have worked with Latinos for years say they routinely face skepticism among families who worry that signing up for insurance will attract immigration authorities, who could move to deport family members living here illegally.

"A lot of the issues boil down to trust — can they trust the information they’re getting and can they trust that what they give will only be used for health care," said Ellen Wu, executive director of the California Pan-Ethnic Health Network. "We hear all the time of families where one parent could help their children enroll, but they fear that the information is going to be sent to immigration authorities. You’ve gone all this time without something, so you’re not going to risk that you lose a family member just to get it."

Immigrants living in the United States illegally are prohibited from buying coverage on the state and federal exchanges and are not eligible for Medicaid. But their American-born children are eligible for federal subsidies or Medicaid, depending on their income. Immigrant advocates say the parents often assume that their children are also ineligible, or fear that they could be deported if they applied for coverage.

Unlike other states, California allows young people who were granted deportation deferrals by the Obama administration to sign up for Medicaid. State Sen. Ricardo Lara is expected to introduce legislation this week that would allow immigrants living here illegally to sign up for the exchange and receive state subsidies.

Even among those living here legally, there is fear that receiving Medicaid or insurance subsidy will jeopardize their chance at citizenship by labeling them a "public charge," a term used to refer to immigrants dependent on the government, experts say. The Obama administration has issued guidelines assuring immigrants that receiving medical assistance will not affect their immigration eligibility, but many health care officials say the guidelines have done little to quell fears and confusion.

Latinos now make up nearly 50 percent of the population in California. Covered California officials and other outside experts have projected that Latinos should account for at least one-third of all those who sign up for coverage through the exchange. California is the only state to release enrollment numbers by ethnicity; the federal health care exchange serving 36 states and the 14 state-based exchanges do not require applicants to identify their race.

Marisa Nuqez first heard about the health care exchange from her daughter’s teacher at school, and she was eager to find a counselor to help answer her questions. But after a week of making calls and being placed on hold for nearly an hour, Nuqez gave up. Only her daughter’s complaints about stomach pains for several days prompted Nuqez to try again, when she discovered she would not be eligible to purchase insurance through Covered California, because she immigrated here illegally.

"There is nowhere I can go for help for me, which is scary," Nuqez said. Although her oldest daughter, who came with her from Mexico a decade ago, is also barred from assistance, her two youngest children, both born in the United States, most likely qualify for Medicaid, an option she is exploring. "I still do not know what my children can get or not," she said. "I want the same thing as everybody else: a place for them to be able to go when they are sick."

For those like Nuqez who have spent years relying on walk-in clinics, cost is also a deterrent. Amanda Aguirre, president of the Regional Center for Border Health in Yuma, Ariz., said many Latinos there felt the private plans available through the exchange were too expensive, even if they qualified for federal subsidies.

"They still feel it’s a very high premium, like it’s another car payment or another mortgage payment, and they will not accept it," Aguirre said. "They go uncovered."

Aguirre said that in border communities like Yuma, many Latinos were accustomed to traveling to Mexico for "very cheap care" and thus did not see the need for insurance. "We’re trying to say, ‘Hey, you don’t have to go across the border,’" she said. Many immigrants who have spent decades without insurance may also have difficulty understanding why they should pay for it now, experts said. "You need someone to sit down and explain why they really need comprehensive care, how they will be able to come into a doctor’s office differently, how they can put this to their use," said Howard Kahn, chief executive officer of LA Care, the largest public health insurance plan in the country and among the most successful in attracting Latino customers.

California has begun taking steps to increase enrollment among Latinos, including adding enrollment counselors to its Spanish-language hotline center. Health plans have also begun advertising on food trucks that cater to Latino neighborhoods, and marketing campaigns are being reformulated to convey a message that officials believe will resonate there.

"I don’t think it’s the lack of dollars that’s the problem here, but there’s obviously something missing in the execution," said Elvia Delgado, vice president for AltaMed, a clinic in eastern Los Angeles that has signed up more than 11,000 patients in Medicaid and Covered California in the past four months. "We need to effectively communicate to people that this is something for them."

Mayra Alvarez, associate director of the Office of Minority Health at the Department of Health and Human Services, said the Spanish version of the federal exchange website, www.cuidadodesalud.gov, was now "fully functional" and that the Obama administration was ramping up efforts to draw Hispanics to it, including running a Spanish-language television ad in heavily Hispanic markets.

"Survey after survey tells us the interest is there, and when you talk to folks and answer their questions, that’s when their interest is even more piqued," she said. "So it’s a matter of getting out there."

Jennifer Medina and Abby Goodnough, New York Times