The Obama administration on Sunday begins what is expected to be the toughest enrollment period yet for ObamaCare.

The administration has sought to lower expectations, trimming back its overall enrollment projection while warning that the people who remain uninsured will be the hardest to reach.

Secretary of Health and Human Services (HHS) Sylvia Mathews Burwell said she and her team are putting affordability at the forefront of the three-month enrollment sprint in an attempt to ease concerns about rising premiums.

“A lot of people don’t actually understand” how ObamaCare works, Burwell told reporters Thursday. “They don’t even know about the tax credits or the financial assistance. Some don’t even know at all, some are confused by it.”

Still, Burwell has acknowledged, “this open enrollment is going to be tougher than last year.”

The administration’s goal of having 10 million people enrolled by the end of next year, up from 9.1 million this year, is widely viewed as a low target that would represent a modest expansion of President Obama’s signature domestic program.

“It isn’t very much growth,” said Karen Pollitz, a former HHS official in the Obama administration who is now at the Kaiser Family Foundation.

While there is uncertainty about how the enrollment period will go, some fear that not enough healthy people will sign up to create a “risk pool” that balances the sick enrollees and prevents premiums from spiking.

“That would be a concern,” Pollitz said. “Sort of, how big of a risk pool do you need?”

Richard Frank, HHS assistant secretary for planning and evaluation, said the target does not mean enrollment growth has “plateaued,” but did say there would be “a much longer path” to signing up the rest of the uninsured.

About 29 million people remain uninsured in the United States, according to the Centers for Disease Control and Prevention. Around 45 million people were uninsured when ObamaCare’s coverage expansion began in 2013.

To try to boost sign-ups, federal health officials are planning to blanket key regions with advertisements that promote “financial assistance” that can reduce the cost of coverage to as little as $22 per month. HHS plans to buy $35 million worth of ads, according to a department official, down from $53 million in the first year of enrollment.

Administration officials are quick to point to national polling that shows almost 60 percent of uninsured people either have not heard about the healthcare law’s tax credits to help people afford coverage, or do not understand how they work.

“You’re going to hear us talk about affordability in ads, in enrollment centers, just about everything we do,” Kevin Griffis, HHS assistant secretary for public affairs, said Friday.

The healthcare law’s tax credits help shield people from premium costs, but there is concern that for middle class people with somewhat higher incomes, who get less of a credit, there is not enough financial help to spur them to enroll.