WASHINGTON — Premiums for midlevel health plans under the Affordable Care Act will increase by an average of 25 percent next year, while consumers in some states will find significantly fewer insurance companies offering coverage, the federal government said Monday.

But the Obama administration said three-fourths of consumers would still be able to find plans for less than $100 a month with the help of federal subsidies.

The open enrollment period under President Barack Obama's signature health law begins on Nov. 1, but consumers got their first look at their options on Monday. Consumers who go without insurance next year could face possible tax penalties of $700 a person or more.

In many parts of the country, the available options are sure to become part of the political conversation in the election season's closing days. And the rising costs and shrinking options all but ensure that the next president will need to make significant adjustments to the health law, something Hillary Clinton and Donald Trump have promised.

The average increase of 25 percent in benchmark premiums on the federal exchange compares with increases of 2 percent in 2015 and 7 percent this year. Major insurers have pulled out of the public marketplace in many states, citing multimillion-dollar losses, and state officials have approved rate increases of 25 to 50 percent or more for some insurers that remain.

One in five consumers on the federal health insurance website HealthCare.gov will find only one insurer with offerings next year, the administration said.

For a 27-year-old consumer, in the prime age group sought by insurers, the average monthly premium for a benchmark plan would be $302 next year, up from $242 this year, according to a report from the Department of Health and Human Services.

"While the president's allies in Washington will try to spin the numbers, families across the country will be forced to figure out how to pay for such unaffordable insurance," said Sen. Orrin Hatch, R-Utah, who is an opponent of the Affordable Care Act and the chairman of the Senate Finance Committee, which has jurisdiction over much of the law.

The administration minimized the marketplace's troubles. Officials said people could lower their costs by seeking income-based subsidies in the form of tax credits and by switching to less expensive plans. While some markets will have few choices, consumers nationwide will see an average of 10 plans per insurer. Plans vary in cost and cover different doctors, hospitals and drugs.

The Obama administration said last week that it expected monthly enrollment in the Affordable Care Act marketplace to average 11.4 million next year, up from a monthly average of 10.5 million this year. But 5 million to 7 million people who buy insurance on their own do not receive federal subsidies

"The number of people eligible for tax credits will increase" as premiums rise next year, and the amount of assistance will also increase, Kevin Griffis, a spokesman for the Department of Health and Human Services, said.

Under the federal health law, insurance plans are classified in four levels: bronze, silver, gold and platinum, depending on the amount of coverage.

"If every returning consumer nationwide selected the lowest-cost plan within the same metal level they picked last year, average premiums — taking into account financial assistance — would fall by $28 per month, or 20 percent, compared with 2016," Kathryn Martin, an acting assistant secretary of health and human services, said.

But in many places, the options have shrunk.

Laura Schlett, 44, of Brandon, Miss., a suburb of Jackson, said she was losing coverage because her insurer, UnitedHealth, was pulling out of the public marketplace there, as in many other states.

Schlett said that she received a subsidy of $110 a month and spent $240 a month for health insurance with a $3,000 deductible this year.

"I can't afford to get sick after paying for the health insurance," Schlett said.

In Phoenix, only one insurer, Health Net, a subsidiary of Centene, is listed on HealthCare.gov, and it is offering only four plans for 2017. A Phoenix family of four with income of $62,000 a year could obtain a premium tax credit of $1,079 a month next year, but would still owe $342 a month, or roughly $4,100 a year, and the policy has an annual deductible of $14,100 for the family, according to HealthCare.gov.

In Raleigh, N.C., 18 plans are available for 2017 from two insurers, but the premiums could be a substantial expense for some consumers. For a 53-year-old man with income of $53,000 a year, the cheapest midlevel silver plan will cost $714 a month, or $8,568 a year, according to the federal website, and no subsidy would be available.

But a man of the same age with income of $25,000 a year could get a subsidy of $639 a month, reducing his premium to $75 a month, or $900 a year.

In Eugene, Ore., the cheapest silver plan available to a 35-year-old man has a full, unsubsidized premium of $348 a month, or $4,176 a year, and an annual deductible of $3,000. A total of 14 plans are available from two insurers.

In Charleston, W.Va., consumers have a choice of 16 plans offered by two insurers. For a 40-year-old woman, the cheapest silver plan has a full unsubsidized premium of $505 a month, or $6,060 a year, and an annual deductible of $6,150. If the woman has annual income of $30,000, she could get a subsidy of $326 a month, but would still owe premiums of $179 a month, or about $2,150 a year.

Stabilizing the insurance marketplace could be a major challenge for Clinton, the Democratic candidate for president, if she wins, as opinion polls suggest she will.

The assurances by the Obama administration on Monday differed in tone from Clinton's remarks on the campaign trail. She has supported the Affordable Care Act, but denounced "skyrocketing out-of-pocket health costs" and said the federal government should have the power to block or modify unreasonable rate increases so coverage would be more affordable.