A durable legal adage warns that “bad cases make bad law.” The U.S. Supreme Court is set to hear arguments today in a flimsy challenge to the Affordable Care Act. Justices should not allow the trumped-up case to deny as many as 13 million Americans, including nearly 375,000 Ohioans, the financial aid they need to buy health insurance.

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Reactionary lobbies that have sought to wreck Obamacare since it became law five years ago are pressing the suit on behalf of four named plaintiffs of dubious standing. The plaintiffs argue bizarrely that they are somehow harmed by their ability under the law to receive tax credits for the health coverage they purchase.

The opponents dispute the legality of the income-based subsidies that the Affordable Care Act offers consumers who buy insurance in the 34 states — including Ohio and Michigan — that rely on the HealthCare.gov exchange run by the federal government, rather than a state-operated exchange. They argue that the subsidies are available only in states that have created their own online exchanges. They base that contention on a single reference in the health-care reform law to exchanges “established by the State.”

Really. That’s pretty much their entire case. And it’s nonsense.

The language of the Affordable Care Act, and the congressional debate that preceded its passage, make clear that lawmakers intended the tax-credit subsidies to be universally available. Among other things, the law specifically cites its goal of “health care for all Americans.” The Internal Revenue Service, in defining regulations to enforce the law, has followed that intent.

The contrary interpretation — that Congress deliberately sought to punish states that did not set up their own exchanges — represents wishful thinking on the part of the plaintiffs, rather than solid grounding in either history or law. That their challenge has made it all the way to the Supreme Court, and that justices agreed to hear it, is troubling.

In past cases, current justices have argued for the constitutional interpretation of laws based on their overall wording and context, not an isolated and arguably ambiguous phrase. They need to apply that standard in this case as well.

If the plaintiffs prevail, the Kaiser Family Foundation — a nonprofit group that analyzes health policy — estimates that next year, 13.4 million lower-income Americans could be denied financial help under Obamacare to pay their health insurance premiums. That projected total includes more than 374,000 Ohio residents and 676,000 Michigan residents.

The Affordable Care Act depends greatly on access to these subsidies by consumers in all states. Gut them, and the law no longer works.

The legal challenge also threatens provisions of the law that require most Americans to buy health insurance, and larger employers to insure their workers, or face a financial penalty. The Supreme Court has upheld the constitutionality of the so-called individual mandate.

Republican lawmakers don’t need another excuse to try to repeal a law they disdain, even though it is helping millions of Americans get affordable health coverage. The law also is lowering costs for consumers with private health insurance, who now must subsidize uninsured patients.

A bare majority of Supreme Court justices upheld the constitutionality of the Affordable Care Act in 2012. Justices need to affirm the law again this year, rather than uphold such an implausible challenge.

First Published March 4, 2015, 5:00 a.m.