The result has been that the population actually signing up for coverage is both smaller and sicker than it would otherwise have been, forcing insurers to charge higher premiums.

But that’s where the subsidies come in.

Under the A.C.A., the poorest Americans are covered by Medicaid, so private premiums don’t matter. Meanwhile, many of those with higher incomes — up to 400 percent of the poverty line, or more than $95,000 for a family of four — are eligible for subsidies. That’s 59 percent of the population, but because many of those with higher incomes get insurance through their employers, it’s 83 percent of those signing up on the exchanges. And here’s the thing: Those subsidies aren’t fixed. Instead, the formula sets the subsidy high enough to put a limit on how high premium payments can go as a percentage of income.

What this means is that of the 27 million Americans who have either gained coverage through the Medicaid expansion or purchased insurance on the exchanges, only about two million are exposed to those Trump-engineered premium hikes. That’s still a lot of people, but it’s not enough to get a death spiral going. In fact, for complicated reasons (“silver-loading” — don’t ask), after-subsidy premiums have actually gone down for many people.

And that leaves the G.O.P. very, very frustrated.

From the beginning, Republicans hated Obamacare not because they expected it to fail, but because they feared that it would succeed, and thereby demonstrate that government actually can do things to make people’s lives better. And their nightmare is gradually coming true: Although it took a long time, the Affordable Care Act is finally becoming popular, and the public’s concern that the G.O.P. will kill it is becoming an important political liability.

What this says to me is that if Republicans manage to hold on to Congress, they will make another all-out push to destroy the act — because they’ll know that it’s probably their last chance. Indeed, if they don’t kill Obamacare soon, the next step will probably be an enhanced program that lets Americans of all ages buy into Medicare.

So voters are right to believe that health care is very much an issue in the midterm elections. It may not be the most important thing at stake — there’s a good case to be made that the survival of American democracy is on the line. But it’s a very big deal.