What will happen if President-elect Donald Trump and Republicans in Congress carry out their pledge to repeal the Affordable Care Act, the 2010 health reform law? By most estimates, up to 22 million people, many of them poor or older Americans, will lose their health insurance.

Mr. Trump seems to recognize this would be disastrous — to an extent. Since the election, he has said that he wants to keep the part of the law that prohibits insurance companies from discriminating against people with pre-existing conditions. Without this provision, insurers can deny those customers coverage, charge them exorbitant rates or refuse to cover treatment for those conditions.

But Mr. Trump and other Republicans are delusional if they think that they can preserve that provision while scrapping the rest of the health care law. Insurers are able to offer policies to people with pre-existing conditions because the law greatly expands the number of people who are insured, thus spreading the costs of treating people with chronic conditions over a larger number of paying customers.

The law provides subsidies to help individuals and families buy policies on government-run online health exchanges (those who do not buy insurance are required to pay a tax penalty). Take away or greatly reduce that benefit and millions won’t be able to afford the plans, and many insurers will simply stop selling policies or will charge individuals and families much higher rates. The law also expanded Medicaid by financing state programs with federal money. Take away that money, and many states will no longer be able to provide care to millions of poor families.