Back in May, the UPMC Health Plan in Pennsylvania said it would need to increase its health insurance premiums on the Affordable Care Act’s marketplace by 8 percent, on average, in 2018.

It was an entirely unremarkable number that would allow the plan to cover the expected increase in health care costs and account for the return of the ACA’s health insurance tax. “For us, our population is starting to stabilize,” Kim Cepullio, president of UPMC commercial products, told me this week.

Then on Monday, UPMC issued a substantial revision. Premiums weren’t going up 8 percent next year after all. Instead, the increase would be 41 percent, on average.

What changed? Obamacare hadn’t been overhauled. The customers were still the same. But President Donald Trump had inherited an individual health insurance market that was starting to stabilize and decided to break it.

After months of threats, Trump announced late last week that he would halt federal payments to health insurers, known as cost-sharing reductions. His administration also slashed budgets for Obamacare outreach and suggested it would not enforce the law’s individual mandate, which requires every American to have insurance or pay a penalty.

Insurers like UPMC have responded by hiking their rates by upward of 40 percent. “The elimination of the CSRs has a big impact on our rates next year,” Cepullio told me.

Trump is not willing to take any blame: “When the premiums go up, that has nothing to do with anything other than the fact that we had poor health care — delivered poorly, written poorly, approved by the Democrats,” the president said on Monday. “It was called Obamacare.”

Over the past two weeks, though, as I spoke with a nearly dozen health insurers, actuaries, and policy experts, there was near-unanimous consensus: Trump was directly responsible for driving premiums higher. If the White House had made a good-faith effort to implement Obamacare, they said, premiums on the marketplaces likely would have increased by single digits on average.

An emerging Senate deal could belatedly try to undo the damage — but 2018 rates have already been set and the plan has no guarantee of passing Congress.

Instead, premium hikes of 40 percent will be commonplace. Some states and health plans are taking steps to mitigate the actual rate hikes customers will feel. Many Americans buying coverage will be protected from these increases, because they receive financial assistance from the federal government. But the US taxpayer will be on the hook to cover those costs.

People who don’t qualify for aid, meanwhile, could bear the full brunt of those hikes — putting a deeper dent in their wallet or making insurance altogether unaffordable.

“The notion that this is a market-driven phenomenon is ridiculous,” said Mario Molina, the former CEO of Molina Healthcare, a big player in the market. “This is Donald Trump monkeying around with health insurance, which is something he doesn’t understand.”

Obamacare was finally starting to stabilize after its first few years

Despite Trump’s frequent insistence that Obamacare is failing, the law’s markets have actually been trending toward some kind of stabilization, insurers and experts said.

Insurance companies had figured out how to price their plans, and their margins were improving. They were making twice as much money on average per customer in 2017 than they were in 2016. The law’s tax subsidies made coverage attractive to the nearly 9 million who qualified for them and used them to buy coverage, capping the monthly premium they would have to pay.

But what the market couldn’t plan for was Donald Trump being elected president.

Over the past year, the Trump White House has sown doubt about whether the law’s individual mandate would be enforced and cut funding for enrollment outreach. Last week, Trump said he would officially end the cost-sharing reduction payments, which compensate insurers for providing discounts to lower-income customers on their out-of-pocket costs. Insurers have long said they would hike premiums to make up for the lost payments if that happened.

The ACA was starting to find its footing. Then Trump knocked it off balance.

“If this was a normal world, where everything was working, you would have single-digit rate increases,” Molina said.

UPMC is a prime example. Its original proposal in May was to raise rates by 8 percent on average — between 3.1 and 13.1 percent, depending on the specific plan.

The carrier had to account for certain variables, like the ongoing increase in health care costs and some uncertainty because other plans had exited the Obamacare markets, which would bring new and unknown customers into its pool. The most notable piece of the initial proposed increase was the return of the ACA’s health insurance tax, which required about a 5 percent bump.

“We’re about able to keep those increases below medical trend,” Adam Pittler, the director of product development at UPMC, told me. “I think carriers have much more experience with this population. The risk pool itself is stabilizing.”

Other insurance experts who systematically analyze the ACA markets shared the same view of what a world without Trump would have looked like.

The story is different in every state and every county — some areas are faring better or worse than others. But the overall picture was one of a stabilizing market.

“We probably would be seeing rate filings for 2018 … between 5 to 10 percent, on average,” David Anderson, a former insurance official who now researches health policy at Duke University, told me.

Dave Dillon, a fellow with the Society of Actuaries, estimated that underlying cost trends would have generally increased premiums by 5 to 10 percent, and the return of the insurance tax would have added another 2 to 4 percent. Oliver-Wyman Health, a consulting firm, estimated a 5 to 8 percent increase based on cost trends and then a 3 percent increase from the insurance tax.

Those factors would have been present no matter what. They’re the fundamentals of the ACA market. But they don’t account for Trump.

“We’ve got this laundry list of things he has done that has clearly made it worse than if Hillary Clinton were president,” said Bob Laszewski, a health care consultant with close ties to the insurance industry.

Why Obamacare premiums were already going up

Why were some increases still expected this year? Health care costs continue to rise, insurers are still adjusting to the market, and outside factors — like the return of the ACA’s health insurance tax after a one-year moratorium — would have driven prices up.

The health care law’s markets for individual health plans certainly had flaws before Trump was elected. The ideal scenario the Affordable Care Act’s architects envisioned — one in which every American who didn’t get insurance through work and Medicaid or Medicare would shop through the law’s exchanges for a plan, creating a robust market of healthy and sick, young and old — didn’t materialize.

The law was very effective in drawing in people with lower incomes, who qualified for generous financial aid that kept their premiums as low as $100 or $200 per month. Of the 10.3 million people who signed up through the Obamacare marketplaces last year, 8.7 million received federal assistance.

And millions of people who were previously excluded from the individual insurance market had a chance to buy health coverage for the first time after the law ended discrimination based on a person’s medical history. That had been an explicit goal for Democrats when they passed the ACA.

But those changes had consequences. Those new customers had a lot of health care they needed or wanted to get. Data from insurers showed that the new ACA enrollees were receiving significantly more medical care.

“We’re all aware there was some pent-up demand within the population — maybe a little more pent-up demand than many carriers expected. A lot of carriers might have underpriced in 2014,” UPMC’s Pittler told me. “But that pent-up demand started to go away a little bit. We educated people on how to utilize health care.”

At the same time, insurers struggled to attract younger and healthier people who would pay in premiums but typically wouldn’t spend as much on care. They were seeing higher rates than they had in the pre-ACA markets, and many experts believe the law’s individual mandate, which was supposed to compel everybody to sign up, was too weak to have the desired effect.

That meant the markets had to adjust, and they did. Plans hiked their prices to cover their costs, knowing that the millions of people who received subsidies would be protected by the law’s design. But for Americans making too much money to get assistance, they faced the full force of those increases.

“We have just not done a good job of getting healthier higher income people into the ACA,” Craig Garthwaite, a conservative health economist at Northwestern University’s Kellogg School of Management, told me. “I have grown increasingly pessimistic that we’ll have the non-group market that I thought the ACA would be.”

So the markets weren’t perfect, and they weren’t working well for many people who didn’t receive any financial aid to offset the premium increases in the initial years.

But Obamacare wasn’t imploding either. It had a committed market made up of millions of people who receive the law’s generous subsidies.

“If your definition of market stability is an efficient pool giving people the lowest possible rate base, we aren’t even in sight,” Laszewski said. “But is there a death spiral? There isn’t a death spiral because the subsidies keep the subsidized people in the pool.”

And after a few years of experience, experts generally agree, insurers had a much better handle on who the ACA customers were and what their care would cost, which should have made price increases going forward more predictable and manageable.

“I think at this point, the experience factor in all of this has improved a lot,” Molina said. “Actuaries feel much more comfortable about who the patients are, what services they need.”

That was the world as it would have been if Hillary Clinton were president or perhaps a Republican less inclined to blow up Obamacare (John Kasich?) were in the White House.

But that is not the world we live in. Donald Trump is president — and he wants Obamacare to implode.

“The biggest difference, I believe, is you would have an administration actively interested in making things work and work over the long term,” Anderson told me.

Or as Garthwaite put it: “Some of the uncertainty would have existed even without Trump. But Trump has definitely exacerbated it.”

How Trump is driving premiums even higher

Trump has sown uncertainty about Obamacare’s future for months. But in recent days, just weeks before ACA enrollment starts, he’s taken to directly attacking the law.

He had threatened since the spring to cut off the cost-sharing reductions payments and, last week, finally did. Insurers are still required to offer cost-sharing discounts to their lower-income customers, so they have increased premiums to protect against Trump stopping the payments that compensate them.

The cost-sharing reductions could have still been an issue due to an ongoing lawsuit, but it’s safe to say a Democratic White House or Senate would have done all they could to ensure the payments were made. With Trump, health plans couldn’t make that same assumption. Congressional action, a bipartisan deal to fund the payments, is probably coming too late to provide CSR certainty for 2018 and bring premiums back down.

“The president has used this as a club, and he has been beating up the industry with this,” Molina said. “It’s an even bigger effect than it would have been because of the president’s behavior.”

Top Trump officials have also hinted the administration might stop enforcement of the individual mandate. The absence of a mandate gives healthier people less of an incentive to sign up for coverage, meaning the people who buy insurance are even more likely than before to be older or sicker.

Lastly, the White House is also slashing funding for enrollment outreach. That, too, hurts the market: Sick people will probably find a way to sign up for insurance no matter what, whereas healthier people need more prodding to shop for coverage and enroll.

Actuaries disagreed about exactly how much Trump is to blame for the rising premiums. It’s an imprecise science. But the consistent story in my reporting was that Trump has substantially added to the premium increases that we’ll see in 2018:

UPMC, for starters, revised its rates from an average 8 percent increase to an average 41 percent increase after Trump stopped the cost-sharing payments. “The elimination of the CSRs is a big number,” Cepullio said.

Dillon, with the Society of Actuaries, attributed a 10 to 20 percent increase to the CSR uncertainty and a 0 to 10 percent increase to concerns about the mandate — the two areas where Trump has most affected the market’s outlook.

Molina pegged the CSR issue to a 20 to 30 percent increase, the doubts about the mandate add another 10 percent, and the outreach cuts contribute a final 5 percent.

Oliver Wyman Health said “up to two-thirds” of the rate increases would be due to the loss of CSRs and the mandate uncertainty.

Frustrated with Republicans in Congress and their inability to repeal the ACA, Trump is taking steps himself to gut it.

But premiums will increase as a result, whether he admits it or not.

“While there clearly were problems with the law’s original design, the market has been stabilizing and insurers are now mostly profitable in the ACA marketplace,” Larry Levitt, senior vice president at the Kaiser Family Foundation, told me. “The problems going forward are now largely a result of uncertainty sowed by the Trump administration.”

Millions of Americans will be protected from those price hikes, due to the health care law’s design. The federal government will have to pick up the tab for them.

But still millions more could be on the hook for the full cost. Some insurers, like UPMC, are expanding the plans they offer off the ACA marketplaces in an attempt to provide that population with more affordable options. But it is the middle-class people making too much to qualify for the law’s subsidies — a single woman making $65,000 a year or a family of four with a $120,000 income — who are most at risk.

That is, at least in part, a failure of Obamacare. But just when the ACA was starting to experience some real stability, Donald Trump decided to shake up the law.