Polls suggest that the public considers health care the most important issue in the midterm elections. This immediately raises the question: Do voters understand what’s at stake? In particular, do they realize that if Republicans hold Congress, they will strip away protections for the 52 million Americans — more than a quarter of nonelderly adults — who have pre-existing conditions that, before passage of the Affordable Care Act, could have led insurers to deny them coverage?

In fact, the Trump administration is already trying to take away those protections via the courts. It probably won’t succeed. But it might, in which case an estimated 17 million Americans would lose their health coverage.

And even if the lawsuit fails, the administration’s support for an incredibly flimsy legal challenge — one so indefensible that three career Justice Department lawyers withdrew from the case — is a clear signal of Republican priorities: G.O.P. to Americans with health problems: Drop dead.

By the way, some people seem surprised by the administration’s moves here, since Donald Trump has promised many times to protect people with pre-existing conditions. But remember: The campaign against the Affordable Care Act has been based on lies every step of the way.

First there were lies about what was actually in the act. Remember “death panels”?

Then there were lies about the law’s effects. For a while, the Koch brothers-financed group Americans for Prosperity was running ads featuring supposedly real stories of Americans facing terrible hardships because of the A.C.A. But none — none — of these stories stood up to fact-checking. So the ads became vaguer and vaguer, and eventually featured actors pretending to be A.C.A. victims rather than featuring real victims, who were apparently too hard to find.

But the most enduring lie from A.C.A. opponents — not just Trump, but all of them — is their claim that they want to protect Americans with pre-existing conditions. They don’t, and they never did.

You can see why they claim otherwise. A huge majority of voters, including 59 percent of Republicans, want to maintain rules that prohibit insurance companies from denying coverage based on someone’s medical history. So there is a powerful incentive to pretend that you’ll protect people with past health problems.

But the falseness of the pretense has always been obvious.

This falsity was obvious on sheer logical grounds even before Republicans began proposing supposed replacements for Obamacare. If you’re going to guarantee coverage regardless of medical history, you have to induce people to sign up for insurance while they’re still healthy, so that insurers have a manageable risk pool. That means some combination of subsidies to make insurance affordable and penalties for going uninsured — in other words, it requires a system that looks a lot like the Affordable Care Act.

So demands that the A.C.A. be scrapped always meant taking away coverage from the people who need it most; Obamacare opponents just hoped people wouldn’t notice that fact. And the truth is that they mostly got away with it until last year, when Republicans had to offer specific health care legislation.

At that point the game was up. It immediately became clear that every Republican alternative to Obamacare would, in fact, hang Americans with pre-existing conditions out to dry. And the public backlash against that revelation is basically the reason the G.O.P.’s repeal effort failed. But it only failed narrowly. And if Republicans still hold Congress next year, anyone who has a history of medical problems and doesn’t get health insurance from his or her employer will lose coverage.

In fact, even getting a job with insurance coverage might not be enough: If the Trump-supported lawsuit succeeds, employers could refuse to cover new employees’ pre-existing conditions.

What may seem puzzling about all this is the cruelty. O.K., Donald Trump is obviously a man utterly lacking in empathy. But don’t other Republicans feel a bit bad about the prospect of taking health care away from millions of Americans who have done nothing wrong besides having past medical problems?

Actually, no. Consider Rick Scott, the governor of Florida (and current Senate candidate), whose attorney general has joined the lawsuit to eliminate protection for pre-existing conditions. While refusing to say whether he supports the suit, Scott declared, “We’ve got to reward people for caring for themselves.” Right, because if you get cancer, or arthritis, or multiple sclerosis — all among the pre-existing conditions for which people used to be denied coverage — it must be your own fault.

By the way, a note to older Florida voters: You may think that none of this matters to you, because you’re covered by Medicare. If so, think again: If Republicans win in November, they’ll be coming after Medicare next, to offset the cost of their tax cut. Who says so? They do.

So, as I said, voters need to understand the stakes in these midterms. They will determine whether people with medical problems get the health care they need.