The latest legal challenge to Obamacare just became very credible—and very dangerous.

The Supreme Court on Friday announced that it would take up King v. Burwell, one of several cases in which plaintiffs say the federal government lacks authority to distribute insurance tax credits in certain states. Unlike the previous challenge to Obamacare, on the constitutionality of the individual mandate, these cases make no similarly lofty claims about liberty or the reach of federal power. They are, instead, about statutory interpretation—how to read some ambiguous language in the text of the law and what members of Congress intended when they wrote it.

But if the constitutional stakes of this case are small, the human stakes are very large. To make health care available to anybody, regardless of pre-existing conditions, the Affordable Care Act sets up new marketplaces, through which people can buy regulated insurance policies and, depending on income, qualify for tax credits worth hundreds or even thousands of dollars a year. Those tax credits are critical. Most people now buying coverage in the marketplaces are eligible for them; without that money, many of these people could not get insurance at all.

Obamacare gives states a choice: They can choose to run their own marketplaces or they can hand that job off to the federal government. According to the lawsuits, when states choose the latter—i.e., to let the feds run the marketplaces—then their residents lose access to the tax credits. Supposedly this is what the law’s architects intended. The hope, according to the lawsuits, was that making subsidies conditional upon states running their own marketplaces would entice them to do the job on their own.

The Cato’s Institute’s Michael Cannon and Case Western University’s Jonathan Adler, who are the lawsuits' original masterminds, say the historical record backs up their argument. Pretty much everybody who actually wrote the law—and pretty much every journalist who covered it, including me—say that is utter nonsense.