The Affordable Care Act deals with the fundamental issue of health care provision in two ways. More than half of the gains in coverage have come from expanding Medicaid — that is, collecting taxes and using the revenue to pay people’s medical bills. And that part of the program is working fine, except in Republican-controlled states that won’t let the federal government aid their residents.

But Medicaid only covers the lowest-income families. Above that level, the A.C.A. relies on private insurance companies, using a combination of regulations and subsidies to keep policies affordable. This has worked well in some places. For example, in California, which has tried hard to make health reform work, the number of people with health insurance has soared, while premiums are still well below expectations.

Overall, however, too few healthy people have purchased insurance, despite the penalty for failing to sign up; this is partly because many of the policies offered have high deductibles, making them less attractive. As a result, some companies have pulled out of the market. And this has left some areas, especially rural counties in small states, with few or no insurers.

No, it’s not a “death spiral” — subsidies keep insurance affordable for most people even if premiums rise sharply, and the Congressional Budget Office believes that markets will remain stable. But the system could and should be improved. How?

One important answer would be to spend a bit more money. Obamacare has turned out to be remarkably cheap; the Congressional Budget Office now projects its cost to be about a third lower than it originally expected, around 0.7 percent of G.D.P. In fact, it’s probably too cheap. A report from the nonpartisan Urban Institute argues that the A.C.A. is “essentially underfunded,” and would work much better — in particular, it could offer policies with much lower deductibles — if it provided somewhat more generous subsidies. The report’s recommendations would cost around 0.2 percent of G.D.P.; or to put it another way, would be around half as expensive as the tax cuts for the wealthy Republicans just tried and failed to ram through as part of Trumpcare.