The Department of Health and Human Services reported Monday that premiums for midlevel health plans on the health law’s federal insurance exchange would rise by an average of 25 percent, but in some cities and states, increases will be considerably higher. Mr. Trump asserted that rates would go up “60, 70, 80, 90 percent” — apparently referring to exorbitant jumps in select markets.
Those increases, however, will be cushioned for most people on the exchanges by government subsidies that will rise with the premiums.
Mrs. Clinton, in an interview with a Miami radio station on Tuesday, said she was committed to making “changes to fix problems” in the health law while reaffirming her alliance with President Obama, whose diverse coalition of supporters is crucial to her electoral strategy.
The higher premiums pose an 11th-hour test for Mr. Trump and Mrs. Clinton in a campaign that has scarcely revolved around policy issues. Mr. Trump has repeatedly struggled to prosecute a political case against Mrs. Clinton, most notably failing to focus in a sustained way this summer on the scathing F.B.I. report on her State Department email.
Whether he can make Mrs. Clinton pay a political price for supporting the Affordable Care Act, and more broadly for championing President Obama’s priorities, will reveal his ability to turn a policy issue into a political weapon at this late stage of the race.
“It needs to be the principal message; you can’t dilute the attack by all the other stuff Trump talks about every day,” said Ed Rollins, a veteran Republican strategist.
For Mrs. Clinton, the problems with the Affordable Care Act could force a reckoning that she had hoped to avoid. As a candidate, she has linked herself more closely to Mr. Obama than any nominee has done with a sitting president in modern times, defending his economic record and praising him for pushing the health law through a sharply divided Congress.
Republicans had hoped that their nominee would force Mrs. Clinton to own the health care law, politically speaking, or at least be forced to defend it, but she has mostly skated past its flaws in cost and coverage. A Kaiser Family Foundation poll in September found that roughly six in 10 adults said the candidates’ plans to address of cost of their health insurance premiums and deductibles would be very important to their vote for president.
Still, parts of the health law are politically popular. The United States has the lowest percentage of uninsured citizens in its history. Because of the Affordable Care Act, insurers cannot deny coverage for a pre-existing medical condition and cannot cap lifetime coverage. Children can remain on their parents’ policies until age 26.
Mrs. Clinton says she wants to improve the law by increasing the subsidies that help cover premiums and allowing more Americans to receive government help.
She also wants to add a government-run insurance option, which she says would increase competition and choice in the marketplaces created under the health law. And she has proposed allowing people younger than 65 to buy in to Medicare.
In her Miami radio interview, Mrs. Clinton said of those insured by the law, “Look, this is a major step forward: 20 million people.”
She added that “I’m sure you noticed, predominantly working people, African-American, Latino people now have access to insurance, but the costs have gone up too much. So we’re going to really tackle that.”
Mr. Trump says he wants to repeal the health law and take more of a free-market approach. He would reduce federal regulation and coverage requirements so insurance would cost — and cover — less. He would not require Americans to have health insurance, as the Affordable Care Act does.
“By failing to denounce it, Hillary Clinton owns it,” Kellyanne Conway, Mr. Trump’s campaign manager, said of the health care act in an interview on Tuesday.
That political attack will not necessarily resonate.
While the problems in the individual insurance market are real, they affect only a small fraction of Americans. In 2015, 49 percent of Americans got health insurance through a job, 34 percent got it through Medicare or Medicaid, and 7 percent got it through the individual market, according to the Kaiser Family Foundation, a nonpartisan research group.
Of the 10.5 million Americans who get health insurance through the Affordable Care Act marketplaces, about 85 percent receive income-based subsidies to defray the cost.
That leaves seven million people buying insurance on their own without subsidies, because their income is too high or they are not aware of the option — a significant number but not a huge voting base.
Even for those seven million, the effect will range widely. For a 27-year-old in the battleground states of Ohio and New Hampshire, the average monthly premium for a benchmark plan will rise next year by only 2 percent. In Indiana, where the health care law is helping to shape a key Senate race, the average monthly premium for a benchmark plan will actually decrease, by 3 percent.
But in tightly contested North Carolina, it will rise by 40 percent, to $446 from $319. In Iowa, it will rise by 25 percent, to $308 from $246. And the biggest increase will hit the new battleground of Arizona, where premiums will rise 116 percent.
Mr. Trump stumbled out of the gate on Tuesday as he sought to bash the health law. Speaking to scores of his own workers at his Miami golf course, he said that “all of my employees are having a tremendous problem with Obamacare” — suggesting that his company doesn’t provide health insurance or else misunderstanding the program. Moments later, he said of his employees, “They’re not worried about their health care because we take great care of people.”
After finishing, Mr. Trump left it to the resort’s general manager to talk to reporters about the confusion and confirm that only a few employees may be insured through the Affordable Care Act.
The problems with the health law have been apparent for some time. Neither the law’s subsidies nor its penalties for those who refuse to buy insurance have persuaded enough young, healthy people to go into the insurance marketplaces it created. That has left a customer pool in some parts of the country that is too sick and too small. Because so many marketplace customers have needed expensive medical care, some insurers have spent more on claims than they have earned in premiums.
Another reason for the volatility in the marketplaces is political. The Obama administration, blocked by Republican opponents in Congress, has paid out only a fraction of the $2.5 billion it owes insurers under a provision of the health law that was supposed to protect them from unexpectedly large losses during their first few years in the marketplaces. Several insurers cited the minimal payments as one of the main reasons they raised rates or abandoned the marketplaces.
Republicans zeroed in on the Phoenix rate increase to press their political case.
“I can’t imagine Arizona voting for Democrats,” Hugh Hewitt, a conservative talk show host, said during an interview with Mr. Trump’s running mate, Gov. Mike Pence of Indiana.
“Yeah, it really is incredible,” Mr. Pence said. “Isn’t it amazing that you had Barack Obama a week ago literally celebrating the launch of Obamacare, and then we find out from his own H.H.S. in the last 24 hours that premiums are going to go up 25 percent across the board?”