A question at the heart of the case is whether the Affordable Care Act’s mandate requiring most Americans to buy health insurance or pay a tax penalty remained constitutional after Congress eliminated the penalty as part of the tax overhaul that Mr. Trump signed in 2017. When the Supreme Court upheld the mandate in its landmark 2012 ruling that saved the law, it was based on Congress’s power to impose taxes.

If the mandate is indeed unconstitutional, the next question is whether the rest of the Affordable Care Act can function without it. In December, Judge Reed O’Connor of the Federal District Court in Fort Worth said it could not and declared that the entire law must fall.

But in late June, the United States Court of Appeals for the Fifth Circuit asked for supplemental briefing from the parties on a third question: whether the Democratic states and House of Representatives even have standing to appeal Judge O’Connor’s ruling. To establish standing, a party has to show it has suffered a concrete injury that a ruling in its favor would redress.