TUSCALOOSA, Ala. — Frances Isbell has spinal muscular atrophy, a genetic disorder that has left her unable to walk or even roll over in bed. But Ms. Isbell has a personal care assistant through Medicaid, and the help allowed her to go to law school at the University of Alabama here. She will graduate next month.

She hopes to become a disability rights lawyer — “I’d love to see her on the Supreme Court someday,” her aide, Christy Robertson, said, tearing up with emotion as Ms. Isbell prepared to study for the bar exam in her apartment last week — but staying independent will be crucial to her professional future.

“The point of these programs is to give people options and freedom,” said Ms. Isbell, 24, whose family lives a few hours away in Gadsden.

The care she gets is an optional benefit under federal Medicaid law, which means each state can decide whether to offer it and how much to spend. Optional services that she and millions of other Medicaid beneficiaries receive would be particularly at risk under Republican proposals to scale back Medicaid as part of legislation to repeal and replace the Affordable Care Act.

Those services include dental care for adults, long-term care for disabled and elderly people living at home, certain therapies that children with disabilities receive in school, prosthetic limbs and even prescription drugs.

The battle over replacing the Affordable Care Act has focused intensely on the future of Medicaid, the state-federal health insurance program for the poor and vulnerable created more than 50 years ago as part of President Lyndon B. Johnson’s Great Society. Much of the debate has centered on Republican proposals to roll back the recent expansion of the program to millions of low-income adults without disabilities.

But the House and Senate bills would also make profound changes to the very nature of Medicaid, shifting it from an open-ended entitlement to a program with strict federal funding limits.

Those changes would have far bigger consequences over time, affecting many more of the roughly 74 million Americans on Medicaid. The threat to optional services may be especially acute in states, like Alabama, that already spend far less than the national average on Medicaid and are averse to raising more revenue through taxes.

“In a poor state like Alabama, you are starting off with a baseline that’s already low,” said James A. Tucker, director of the Alabama Disabilities Advocacy Program, which provides legal services to people with disabilities here. “There’s a fundamental antipathy to spending the public purse on health care services for poor people, and that would only get worse if the resources become capped and more limited.”

The drain on Medicaid funding would worsen over time under the bill that Senate Republicans are working to pass, with the new funding limits starting in 2021 and having the biggest impact more than a decade from now. The nonpartisan Congressional Budget Office estimated that Medicaid spending would be 26 percent lower under the Senate plan than it would be under current law in 2026 — and 35 percent lower in 2036. The office predicted that states would be forced to “eliminate optional services, restrict eligibility for enrollment or adopt some combination of those approaches.”

Under the Senate plan, states would receive a fixed annual amount for each Medicaid beneficiary, with each category of beneficiaries, like children and the disabled, getting a different base amount based on recent costs. The amount would increase every year by a formula that is expected to grow more slowly than average medical costs after 2025. Disabled children would not be subject to the spending caps.

Avalere Health, a consulting firm, estimated in a report that federal spending on individual state Medicaid programs could decline between 6 percent and 26 percent under the Republican plan by 2026. The biggest drops would be in states that expanded Medicaid, but the cuts would compound more sharply for every state in later years.

Conservatives say Medicaid spending, which consumes a major and growing portion of the federal and states’ budgets, needs to be reined in. The current system of unlimited federal matching funds, they say, has encouraged states to milk as much as they can, sometimes wastefully; capping funding, their argument goes, would make Medicaid more efficient and ensure it can continue to help the most vulnerable Americans, including people with disabilities.

“The fiscal sustainability of Medicaid is essential to making sure that those who depend on the program can know it will be there for them in the future,” Avik Roy, a conservative health care analyst, wrote last week in Forbes.

Alabama’s two Republican senators, Richard Shelby and Rodney Strange, have expressed quiet support for the Senate bill, although Mr. Strange has said he wants to make sure that “the most vulnerable in my state” are protected. Gov. Kay Ivey, also a Republican, has said she wants to see a final version of the bill before weighing in.

For decades, the only type of long-term care that disabled Medicaid beneficiaries could receive was in nursing homes. But starting in the early 1980s, Medicaid began providing the option of “home and community-based services,” allowing people with disabilities to stay in their homes with paid help. States now spend slightly more for these services than for nursing home care, which is a mandatory benefit under Medicaid.

Nationally, almost three million people received Medicaid long-term care services at home or in their community in 2013, according to the Kaiser Family Foundation. About 13,000 people were getting these services in Alabama in 2015, according to the state.

Medicaid pays for only 25 hours a week of help for Ms. Isbell. It is not enough; she received an additional 25 hours of help through a Department of Education program during her three years in law school.

“It’s still significantly less than I would get in other states,” she said this past week eating a lunch of leftover spaghetti in her apartment in Tuscaloosa with the help of her aide.

She knows disabled people who choose to get catheters, she said, rather than do “pee math”— figuring out how soon they will need to use the bathroom and whether an aide will be there to help.

“People are struggling so much to get by with the hours they have as it is,” she said. “But the way this bill was written, you have to just make a general prediction that these services will be cut.”

Relief for Family

Eric Harkins will never be able to have a job. With cerebral palsy, intellectual disability and a seizure disorder, he cannot speak or move other than scooting across the floor on his knees and elbows. But Medicaid has allowed his sister, Kimberlee, to pursue a career as a vocational rehabilitation counselor instead of caring for him full time. It pays for aides to care for Mr. Harkins for 125 hours a week, an amount that was increased after his mother had a debilitating heart attack and underwent surgery several times over the last few years.

“He requires help with every aspect of daily living,” Ms. Harkins said, stroking her brother’s arm as he watched a cartoon in their living room one recent afternoon in Vestavia Hills, outside Birmingham. “If our caregivers went away tomorrow, I’d have to quit my job and take care of Eric.”

Mr. Harkins, 33, likes playing with toys meant for toddlers, watching shows on his iPad and going on outings to Target and restaurants, though it usually takes his sister and an aide to get him there. Because he can be physically aggressive, a day program is out of the question, Ms. Harkins said. But his mother, Judy Harkins, said that if he was placed in a nursing home, “he would die, and it would kill me, too.”

An Independent Life

Every weekday, Medicaid allows Matthew Foster to spend a few hours pursuing one cherished activity after the next: working out at the gym, taking an art class, shopping for groceries, visiting his elderly aunt. The program pays for an aide to spend 20 hours a week with Mr. Foster, 34, who has Down syndrome and cannot read well or drive.

Mr. Foster spent eight years on a Medicaid waiting list to get the coverage. Before he got it, help came from his mother, Susan Ellis, and his two younger siblings, who have since moved away. His father, Michael Foster, worked six days a week as a coal miner, though he retired recently. Both parents are 67 and have health problems of their own, although Ms. Ellis still works for a nonprofit disability rights group.

Matthew Foster lives in what used to be their garage in a modest split-level home in Vestavia Hills. His living quarters has its own entrance. Since has was 17, he has worked on weekends at Chuck E. Cheese’s, dressing in costume and entertaining children at birthday parties. His father drives him back and forth now, but in the future he may rely on Medicaid for help getting to work.

“My hope is that when Mike and I aren’t around anymore, he will be able to maintain his life the way he lives it now,” Ms. Ellis said. “That means living in the community he’s grown up in.”

As she spoke, Matthew Foster emerged in workout clothes, smiling toward his personal care assistant, Amancia Carrera, who was waiting to take him for their regular Monday sandwich at Subway — “It’s the best time for us to really talk, with eye contact,” Ms. Carrera said — and the gym.

“She’s an active person, and I’m real active, too,” Mr. Foster said of Ms. Carrera, who also works with him on basic math and reading and helps him with housekeeping, meals and bills. “I like having Amancia in my life.”