Judge Carolyn Dineen King was a voice of reason on Wednesday as her fellow members of a three-judge appeals panel sustained a legal threat to the Affordable Care Act.

In a 2-to-1 ruling, the United States Court of Appeals for the Fifth Circuit agreed with Judge Reed O’Connor of Federal District Court for the North District of Texas that the law’s requirement that Americans buy health insurance is unconstitutional. The majority ruling did not accept Judge O’Connor’s reasoning that the mandate’s unconstitutionality made the rest of the law void. But the decision kept that threat alive by sending the case back to him to clarify his reasoning as to which parts of the law, along with the mandate, must be struck down.

Judge King assailed her colleagues for not rejecting what she called “textbook judicial overreach.”

If you’re wondering why the courts are considering the legality of the Affordable Care Act after the Supreme Court already determined it was the law of the land, you have Donald Trump and congressional Republicans to thank.

Mr. Trump’s 2017 tax cut bill reduced to zero the law’s penalty for not purchasing health insurance. Texas then led a group of Republican-led states in claiming that without the penalty the mandate could not survive, since the court had only upheld the mandate as a tax that Congress was entitled to enact. Without the mandate, they claimed with the thinnest of reasoning that the whole law — with its insurance exchanges, Medicaid expansion and coverage for pre-existing conditions — must fall. Mr. Trump’s Justice Department made matters worse when it agreed with the challengers that all of Obamacare should go. As many as 21 million people could lose insurance if the law is killed. Limits on how much insurers cover in a year or a lifetime would end.