President Trump’s sweeping domestic policy law fundamentally alters how food stamps are funded, leaving states to bear some of the cost for the first time in six decades.

The move all but ensures that states across the country will adopt a patchwork of policies for a crucial aspect of the social safety net. Whether they can find the money in their budgets — estimated, in some cases, to surpass a billion dollars — will have wide-ranging, and likely uneven, consequences for some 42 million people who participate in the Supplemental Nutrition Assistance Program, or SNAP.

Some states warn that the entire program is at risk of being eliminated if legislatures cannot come up with their share. Others are weighing whether to impose additional state taxes or cut costs elsewhere in their budgets. And some are considering imposing barriers that will all but certainly reduce the number of recipients on their rolls.

“Even states that are going to try to do their best are going to face really difficult choices. And unless this is undone, it really is the end — and I’m not being dramatic, it’s a reality — of SNAP as we know it,” said Gina Plata-Nino, interim director for SNAP at the Food Research and Action Center, a nonprofit that supports anti-hunger programs.

Under the new law, states are required to cover 5 percent to 15 percent of food stamp benefits beginning in 2028 if their error rate in administering benefits exceeds 6 percent. Higher error rates result in higher cost shares, though the states with the highest rates are given more time to comply. Based on rates from the 2024 fiscal year, all but eight states would be required to pay for benefits.

Altogether, the Congressional Budget Office estimated that states would contribute about $35 billion toward benefits from 2028 to 2034. Coupled with another provision increasing states’ portion of administrative expenses from 50 percent to 75 percent, two-thirds of states will have their share of annual SNAP costs increase by $100 million or more, based on the latest error rates, according to one estimate.

Left unclear is what happens if a state cannot afford their cost share or how the new requirements work in tandem with existing penalties. While some states are waiting for additional guidance from the Agriculture Department, others are beginning to act.

In late August, Gov. Jared Polis of Colorado signed a bill that amended a ballot proposal to include funding for SNAP. The measure, which voters will consider in November, will tax high-income earners to pay for free school meals and, if there is excess revenue, fund the SNAP program.

Others still are warning that they may have to cut enrollment or end their program entirely.

North Carolina’s Department of Health and Human Services said that if local lawmakers were unable to allocate the necessary funding — $420 million for benefits and $14 million for administrative costs — the state would “have to stop offering the SNAP program altogether,” eliminating benefits for 1.4 million residents.

Alabama’s commissioner of human resources recently testified to the state legislature that her agency was considering ending a program that simplifies the application process for older SNAP recipients.

Oregon announced that it would assemble a team to bring down its error rate. A state official who spoke on the condition of anonymity to disclose information that is not publicly available said the state was discussing investing in technological improvements as well as increasing the frequency of income verification — a move that will improve accuracy but also reduce SNAP enrollment.

Republicans in Congress have describedthe cost-share provision as giving states “skin in the game,” asserting that doing so holds the states accountable for taxpayer dollars. But state officials and some experts have argued that financial penalties and corrective processes already exist, and that the way the federal government calculates the payment error rate is flawed.

For example, Ms. Plato-Nano said, something as straightforward as missing a filing deadline could count as part of the error rate calculation, even if an application correctly assessed benefit amount and eligibility.

In an interview, Sarah M. Adelman, New Jersey’s commissioner for human services, said the methodology “artificially inflates” the error rate, so the state is appealing with the Agriculture Department. But beyond the appeals process and working to bring down its error rate, “states really don’t have levers where we can make adjustments to prepare for the shift in benefit cost,” she said. “It’s purely a budget question.”

While she was confident that the governor and legislature would prioritize finding the funding and avoid eliminating SNAP altogether, Ms. Adelman said that states were still ill-prepared for the changes.

“Anywhere north of $100 million is just going to be incredibly challenging for states to identify in their budgets,” she said

More than a dozen state agencies responsible for enacting SNAP said they were working to reduce their error rates to minimize the effect of federal cuts. Some are investing in training staff, public outreach and technologies like automation software and artificial intelligence to process applications and detect inaccurate paperwork. Whether those strategies will be enough remains to be seen.

SNAP error rates rose during the coronavirus pandemic as state agencies faced staffing shortages and the federal government waived certain requirements, said Cindy Long, who served as the Agriculture Department’s Food and Nutrition Services administrator under the Biden administration.

While there was no “magic bullet” to reducing error rates, Ms. Long said that states should be able to do so with adequate staffing levels, training and technology updates.

But she said it was difficult to predict how many states could meet that benchmark and questioned if such a goal made sense. “I don’t know whether 6 percent is the magic number that it’s reasonable to hit and maybe not reasonable to expect them to get below with available resources,” she said.

Some state officials and experts said that such a target was unrealistic. A spokeswoman for Illinois’s Department of Human Resources said the provision was “intentionally designed as another craven mechanism to deny states funding and feed fewer people in need.” The Oregon official argued that the methodology used to calculate error rates sets states up to fail.

Officials also noted that the law was cutting federal resources while asking states to enact additional changes, like expanding work requirements and revoking eligibility for some authorized immigrants like refugees.

“States can’t keep this up,” Ms. Plato-Nano said. “Revenue is not coming out of the sky. So states who have made really great steps forward to make this program more accessible are going to restrict things. Food insecurity is going to rise. Poverty is going to rise. Healthcare issues are going to rise.