But people like Laura Hillenbrand have managed to earn their way out of the coverage gap.

Ms. Hillenbrand, 28, earned $11,000 working part time at a wholesale bakery and a farmer’s market here in 2013. She listed that amount when she first applied for health insurance a year ago, and was deemed ineligible for subsidies because her income was under the federal poverty level of $11,670 for a single person.

But Ms. Hillenbrand wanted insurance so much — she rides her bicycle everywhere, she said, and worries about getting hurt — that she investigated further. After talking with a Foundation Communities counselor, she determined that she could add enough hours at the bakery to get above the threshold. Now she pays about $53 a month for her plan, with a subsidy of $141.

“I needed to earn more money in general, and I’m glad I could,” Ms. Hillenbrand said, adding that she had gone through a divorce and faced higher housing expenses. “But it’s pretty ridiculous that if you don’t make enough, you don’t get Obamacare.”

Some people are avoiding the gap through sheer luck. Deysi Torres, 20, a waitress who was recently reassigned to a busier shift, estimates that she will make $18,000 this year — about $2,300 above the subsidy threshold for her household of two. On her old shift, she said, she made $14,000 a year and would not have qualified. Ms. Torres smiled one recent afternoon as she enrolled in a Humana plan that will cost $28 a month, with a subsidy of $86.

Ms. Colvin said it was particularly hard for low-wage workers to bring their incomes above the threshold if they have several children, because the poverty level is higher for larger families. It is $27,910 for a family of five, for example.

“To get above 100 percent may be just way out of their reach,” Ms. Colvin said.

Children in low-income families, even in non-expansion states, are often eligible for Medicaid even when their parents are not. And even states with stringent Medicaid rules typically allow pregnant women and adults deemed severely disabled to receive Medicaid.