REPUBLICANS LAST week kicked off their dominance of Washington by vowing to push through an unpopular and unwise unraveling of the Affordable Care Act, an imperfect law that nevertheless has done much good. Scaling back the policy is “the first order of business,” Vice President-elect Mike Pence promised after a strategy meeting on Capitol Hill. At the same time, House Speaker Paul D. Ryan (R-Wis.) pledged that Republicans would not pull the rug out from under people currently benefiting from the plan.

How can Republicans keep both promises? Better not to ask them. They seem to have no clue.

Following last year’s election, Republicans first floated the idea of “repeal and delay” — that is, formally canceling large pieces of the law while delaying the phaseout for perhaps several years, giving Congress time to pass a replacement. At first blush, this sounds reasonable (if wrongheaded). It is in fact unworkable.

The ACA depends on private insurers participating in competitive state insurance marketplaces. Without government incentives, and with no reason to believe that their time and effort will pay off under a nebulous new policy down the road, insurers will not continue serving markets that are in any case set to disappear. To avoid a repeal-and-delay disaster, Republicans would have to pour money into Obamacare, a move they ardently opposed when the goal was fixing the program rather than tearing it down. Bottom line: Without a replacement plan passed and in place at the time of repeal, policy uncertainty will drive insurers to quit the markets and desert their patients.

Despite agitating for repeal for the past half-decade, Republicans have failed to unite around any Obamacare alternative, and they do not appear close now. Detailed proposals that have circulated among Republicans over the past several years would almost certainly result in a much skimpier system — covering fewer Americans and degrading the quality of coverage for low-income and sick people who manage to buy it.

Republicans are bumping into some awkward facts, where ideology cannot repeal basic logic. There is no health-care reform that will lower premiums, cut deductibles and increase choice all at the same time, despite President-elect Donald Trump’s rhetoric.

Moreover, an insurance-based health-care system requires pooling many people together so that premiums from healthy people offset the costs of treating the sick — and keep costs reasonable for everyone. Fiddling with regulations that compel people to buy insurance, allowing insurers more room to discriminate against older or sicker people, reducing benefits requirements — all of these could give insurers more opportunity to welcome healthy people and deter the sick, to the benefit of their pocketbooks but the detriment of society as a whole.

GOP leaders are right that health-care costs must be reined in. But they are wrong that market forces will be enough to drive efficiency: Patients — and even doctors and hospitals — often lack critical information and expertise to make appropriate cost-saving decisions. Government must press reforms in how health care is delivered and how the costs and benefits of treatments are evaluated. At some point, people cannot have subsidized access to treatments that are unnecessary or if cheaper ones are just as good.

To a degree, the Affordable Care Act reflects these principles. The GOP alternative proposals floated so far do not. The only way for Mr. Ryan to keep his promise is for Republicans to shore up the system, not engage in repeal and . . . whatever.