Health care is suddenly front and center in the final sprint to the presidential election, and the outcome will shape the Affordable Care Act and the coverage it gives to more than 40 million people.

Besides reproductive rights, health care for most of the campaign has been an in-the-shadows issue. However, recent comments from former President Donald Trump and his running mate, Ohio Sen. JD Vance, about possible changes to the ACA have opened up Republicans to heavier scrutiny.

More than 1,500 doctors across the country recently released a letter calling on Trump to reveal details about how he would alter the ACA, saying the information is needed so voters can make an informed decision. The letter came from the Committee to Protect Health Care, a national advocacy group of physicians.

“It’s remarkable that a decade and a half after the ACA passed, we are still debating these fundamental issues,” said Larry Levitt, executive vice president for health policy at KFF, a health information nonprofit that includes KFF Health News. “Democrats want to protect people with preexisting conditions, which requires money and regulation. Republicans have looked to scale back federal regulation, and the byproduct is fewer protections.”

The two parties’ tickets hold starkly different goals for the ACA, a sweeping law passed under President Barack Obama that set minimum benefit standards, made more people eligible for Medicaid and ensured consumers with preexisting health conditions couldn’t be denied health coverage.

Vice President Kamala Harris, who previously backed a universal health care plan, wants to expand and strengthen the health law, popularly known as Obamacare. She supports making permanent temporary enhanced subsidies that lower the cost of premiums. And she’s expected to press Congress to extend Medicaid coverage to more people in the 10 states that have so far not expanded the program.

Trump, who repeatedly tried and failed to repeal the ACA, said in the September presidential debate that he has “concepts of a plan” to replace or change the legislation. Although that sound bite became a bit of a laugh line because Trump had promised an alternative health insurance plan several times during his administration and never delivered, Vance later provided more details.

He said the next Trump administration would deregulate insurance markets — a change that some health analysts say could provide more choice but erode protections for people with preexisting conditions. He seemed to adjust his position during the vice presidential debate, saying the ACA’s protections for preexisting conditions should be left in place.

Such health policy changes could be advanced as part of a large tax measure in 2025, Sen. Tom Cotton (R-Ark.) told NBC News. That could also open the door to changes in Medicaid. Conservatives have long sought to remake the health insurance program for low-income or disabled people from the current system, in which the federal government contributes a formula-based percentage of states’ total Medicaid costs, to one that caps federal outlays through block grants or per capita funding limits. ACA advocates say that would shift significant costs to states and force most or all states to drop the expansion of the program over time.

Democrats are trying to turn the comments into a political liability for Trump, with the Harris campaign running ads saying Trump doesn’t have a health plan to replace the ACA. Harris’ campaign also released a 43-page report, “The Trump-Vance ‘Concept’ on Health Care,” asserting that her opponents would “rip away coverage from people with preexisting conditions and raise costs for millions.”

Republicans were tripped up in the past when they sought unsuccessfully to repeal the ACA. Instead, the law became more popular, and the risk Republicans posed to preexisting condition protections helped Democrats retake control of the House in 2018.

In a KFF poll last winter, two-thirds of the public said it is very important to maintain the law’s ban on charging people with health problems more for health insurance or rejecting their coverage.

“People in this election are focused on issues that affect their family,” said Robert Blendon, a professor emeritus of health policy and political analysis at Harvard. “If people believe their own insurance will be affected by Trump, it could matter.”

Vance, in a Sept. 15 interview on NBC’s “Meet the Press,” tried to minimize this impact.

“You want to make sure that preexisting coverage — conditions — are covered,” he said. “You want to make sure that people have access to the doctors that they need, and you also want to implement some deregulatory agenda so that people can choose a health care plan that fits them.”

Vance went on to say that the best way to ensure everyone is covered is to promote more choice and not put everyone in the same insurance risk pool.

Risk pools are fundamental to insurance. They refer to a group of people who share the burdens of health costs.

Under the ACA, enrollees are generally in the same pool regardless of their health status or preexisting conditions. This is done to control premium costs for everyone by using the lower costs incurred by healthy participants to keep in check the higher costs incurred by unhealthy participants. Separating sicker people into their own pool can lead to higher costs for people with chronic health conditions, potentially putting coverage out of financial reach for them.

The Harris campaign has seized on the threat, saying in its recent report that “health insurers will go back to discriminating on the basis of how healthy or unhealthy you are.”

But some ACA critics think there are ways to separate risk pools without undermining coverage.

“Unsurprisingly, it’s been blown out of proportion for political purposes,” said Theo Merkel, a former Trump aide who now is a senior research fellow at the Paragon Health Institute, a right-leaning organization that produces health research and market-based policy proposals.

Adding short-term plans to coverage options won’t hurt the ACA marketplace and will give consumers more affordable options, said Merkel, who is also a senior fellow at the Manhattan Institute. The Trump administration increased the maximum duration of these plans, then Biden rolled it back to four months.

People eligible for subsidies would likely buy comprehensive ACA plans because — with the financial help — they would be affordable. Thus, the ACA market and its protections for preexisting conditions would continue to function, Merkel said. But offering short-term plans, too, would provide a more affordable option for people who don’t qualify for subsidies and who would be more likely to buy the noncompliant plans.

He also said that in states that allowed people to buy non-ACA-compliant plans outside the exchange, the exchanges performed better than in states that prohibited it. Another option, Merkel said, is a reinsurance program similar to one that operates in Alaska. Under the plan, the state pays insurers back for covering expensive health claims, which helps keep premiums affordable.

But advocates of the ACA say separating sick and healthy people into different insurance risk pools will make health coverage unaffordable for people with chronic conditions, and that letting people purchase short-term health plans for longer durations will backfire.

“It uninsures people when they get sick,” said Leslie Dach, executive chair of Protect Our Care, which advocates for the health law. “There’s no reason to do this. It’s unconscionable and makes no economic sense. They will hide behind saying, ‘We’re making it better,’ but it’s all untrue.”

Harris, meanwhile, wants to preserve the temporary expanded subsidies that have helped more people get lower-priced health coverage under the ACA. These expanded subsidies that help about 20 million people will expire at the end of 2025, setting the stage for a pitched battle in Congress between Republicans who want to let them run out and Democrats who say they should be made permanent.

Democrats in September introduced a bill to make them permanent. One challenge: The Congressional Budget Office estimated doing so would increase the federal deficit by more than $330 billion over 10 years.

In the end, the ability of either candidate to significantly grow or change the ACA rests with Congress. Polls suggest Republicans are in a good position to take control of the Senate, with the outcome in the House more up in the air. The margins, however, will likely be tight. In any case, many initiatives, such as expanding or restricting short-term health plans, also can be advanced with executive orders and regulations, as both Trump and Biden have done.

Stephanie Armour: sarmour@kff.org, @StephArmour1