THE AFFORDABLE Care Act’s market­places open Tuesday, even though the federal government has shut down. A lot of people don’t know what the opening means, let alone how the broader law is supposed to work. The political debate doesn’t help: It has become increasingly distorted since the law passed three years ago, culminating in inflated Republican claims in recent days that the system will harm, rather than improve, health care for many Americans. That’s not only bad for President Obama’s signature policy achievement, it’s also bad for the health-care system.

To state the obvious, Obamacare is not a panacea. But it’s also not an outrageous government takeover — and it’s likely to help many more Americans than it hurts.

The Affordable Care Act prescribes that everyone should be covered by health insurance, and that everyone should pay something for the benefits they get, if they can. If people don’t obtain insurance from their employers, they have to buy coverage that offers preventative care and other basic services, with the policies sold by companies that can’t reject patients based on preexisting conditions. If those people don’t make much money, the government will help pay their premiums. And more of the poor will qualify for Medicaid coverage, depending on the state.

Fair-minded critics on the left don’t like this arrangement much because they would prefer a single-payer system in which the federal Treasury paid for all care and cost control came from regulation by experts. Fair-minded critics on the right don’t like the Affordable Care Act because it devotes federal money to pay for health care rather than to paying down the debt, or because they don’t like the idea of the government requiring people to buy insurance.

The loudest voices recently, however, have belonged to unreasonable critics: unions that object to the end of government subsidies; and demagogues, such as Sen. Ted Cruz (R-Tex.), who claim that employees and spouses who now are insured will be pushed off their plans, companies will avoid full-time hires and individuals will lie about their incomes to get subsidies they don’t deserve. In fact, most economists expect these effects will be small, if they happen at all.

Apart from GOP obstructionism, the biggest threat to Obamacare may be the still-distinct possibility that not enough people will buy insurance, even with government help. If only the oldest and sickest enter the new insurance system, costs to the government and to customers who don’t get government subsidies could be higher than estimated.

The critical push to sign up lots of people begins Tuesday. The new marketplaces offer coverage options and government subsidies to the uninsured, the under-insured and those who previously bought their own coverage. Americans will have until Jan. 1 to make sure they have insurance.

As with any big rollout, there will no doubt be problems, many of them mundane. Computer systems will not work perfectly. Some people might have to sign up over the phone or on paper. But everyone should hope that those sorts of problems — and the overheated rhetoric of critics — do not deter too many people from buying insurance. Many Americans’ health depends on it.