A new ad aims to encourage uninsured Americans to sign up for health coverage. (Department of Health and Human Services)

The insurance exchanges created under the Affordable Care Act are scheduled to open for a third sign-up season Sunday, with the Obama administration setting modest enrollment expectations and focusing its energies on a niche of people who remain uninsured.

The impending open-enrollment will be devoid of the movie stars and basketball heroes who served as White House megaphones two years ago, when the online marketplaces debuted.

Instead, White House officials have convened 30 phone calls of “stakeholders” across the country on whom they will rely for on-the-ground enrollment efforts. Senior presidential aides have talked with African American religious leaders, including the Rev. Al Sharpton, and hosted a Twitter chat to coincide with Hispanic Heritage Month, according to White House spokeswoman Katie Hill.

Federal health officials have fresh research providing what they consider to be a clearer picture of how many uninsured Americans are eligible to buy health plans through the exchanges, their demographics and where they tend to live.

Because of this research, for example, health officials now know that while a disproportionately large group of Hispanics eligible for ACA coverage remain uninsured, fewer of them speak Spanish than previously assumed. That is why one of several television ads promoting HealthCare.gov, which will air nationally beginning on Monday, will feature a Hispanic chef and a waitress talking up the merits of getting insured — in English.

On Thursday, Health and Human Services Secretary Sylvia Mathews Burwell previewed the new ads for reporters, but she declined to say how the administration’s investment will compare with the $52 million it spent on advertising during the first enrollment season.

HHS is also setting up storefronts, staffed by contractors, to provide enrollment help in 19 cities. It has posted ads in each city, including, for the first time, in laundromats.

Burwell said the administration is trying to spread a three-part message: Health plans are affordable if customers comparison shop; HealthCare.gov, the federal enrollment Web site, is now easier to use; and in-person assistance is available to explain to consumers how health insurance works and to help them select a plan.

She reiterated that the administration is focusing on 10.5 million uninsured people — slightly fewer than one-third of all U.S. residents without coverage — who qualify for health plans through the exchanges. Of that group, she said, “We are going to seek to enroll about 1 in 4.”

Under the 2010 health-care law, the exchanges were formed to sell insurance to people who do not have access to affordable health benefits through a job. The law also created government subsidies to help people pay for these ACA plans, with the amount depending on an individual’s income. Nearly 9 out of 10 people who have gotten this coverage are receiving a subsidy, with enrollment more common among people with relatively low incomes.

The enrollment period that begins Sunday is scheduled to last three months. In 38 states, consumers will rely on HealthCare.gov. The remaining states and the District have created their own exchanges.

At the end of the first open-enrollment season in spring 2014, 8 million people had signed up for coverage despite massive computer dysfunction that had marred HealthCare.gov early on. During the second season, nearly 12 million people got or renewed coverage. Each time, enrollment has slid down as the year went along.

By the end of 2016, Burwell announced this month, HHS expects 10 million people to be covered. That is a negligible increase from enrollment numbers over the summer and minimally higher than the 9.1 million people HHS expects to have ACA health plans by December.

Caroline Pearson, senior vice president of Avalere, a health-care consulting firm, said the administration’s mission for this third sign-up should be to increase enrollment as much as possible. If participation is widespread, she said, insurers will be motivated to remain in the exchanges, and monthly premiums will be less likely to spike.

“The administration releasing really low enrollment targets throws some cold water on that goal,” Pearson said. And by setting a low bar, she added, “They don’t have to pull out all the stops.”

Some of the administration’s chief allies in this effort are also shifting strategies. At Enroll America, the staff has been winnowed from 230 at the start of last year’s open-enrollment to about 150. At the same time, Anne Filipic, the group’s president, said it has expanded its partnerships with like-minded national and local organizations from 4,600 a year ago to about 6,700.

Like the administration, some of these groups are focusing on niches of people who need insurance. This year, the Farmworkers Project in North Carolina began sending teams to help sign up foreigners with work visas who arrived for seasonal crops. The Georgia Restaurant Association is sending notices to all 6,500 of its members, encouraging hourly restaurant workers to make appointments for in-person help at getting coverage.

“We know it is going to be harder” to entice people who have ignored or rejected the opportunity to buy ACA health plans the past two years, Burwell said. “But we need to work smarter.”