By Stephen Dinan

The congressional Democrats involved in writing the Affordable Care Act told the Supreme Court on Wednesday that they intended for consumers in all states to be eligible for subsidies, and they argue that the law they drafted reflects that intention.

Opponents have challenged the Obama administration’s enforcement of the law, arguing that it only allows subsidies to be paid in states that set up their own exchanges, but not to customers in most of the country who use the federal exchange.

Senate Minority Leader Harry Reid, House Minority Leader Nancy Pelosi and their colleagues disputed that in a new brief filed with the Supreme Court Wednesday, saying to leave out some states would defy the entire reason they passed Obamacare — “to make health insurance affordable for all Americans.”

Known as King v. Burwell, the case could poke a giant hole in Obamacare, ruining the economic assumptions that the law relies on.

On Wednesday, House Republicans who oversee health issues wrote a letter to the Health and Human Services Department warning it to begin preparing in case the Supreme Court rules against it.

“Given HHS’s responsibilities, we believe it is prudent that the department plan for the full range of potential outcomes and consequences of the court’s decision,” the GOP lawmakers wrote.

The court case rests on language in the law that says subsidies will be paid to those in exchanges “established by the state.”

Mr. Obama’s opponents say that means the federal exchange, which covers the two-thirds of states that refused to set up their own exchanges, are not eligible. The opponents, pointing to comments from some Obamacare defenders, say Congress had intended to use the subsidies as a way to entice states to set up exchanges, rather than leave the job to federal officials.

But the Democratic lawmakers who wrote and passed the law called that argument “so weak” and say they never meant to draw that kind of distinction, saying the law grants subsidies to “applicable taxpayers” and defines them by income, not by geography.

“It would make no sense to hide such an important condition in such an obscure subsection,” the lawmakers said in their brief, which was filed for them by the Constitutional Accountability Center.

Part of the problem Democrats face is the convoluted way the law was passed.

Senate Democrats forced their bill through on Christmas Eve in 2009, expecting to eventually work out differences with the House, which passed a different bill. But Scott Brown’s victory in a special Senate election in Massachusetts cost Democrats their filibuster-proof majority, and Mr. Reid and Mrs. Pelosi had to resort to using budget gimmicks to force the bill through exactly as it passed the Senate, meaning they didn’t have time to tweak the law, fix drafting errors or make other corrections.

Opponents have pointed to former Sen. Ben Nelson, a Democrat who was one of the key final votes to getting the bill through the Senate in that Christmas Eve vote and who insisted that states be able to set up their own exchanges. The opponents say the subsidies were created as an incentive to get states to set up exchanges.

But Mr. Nelson wrote a letter dated Tuesday saying he “always believed that tax credits should be available in all 50 states regardless of who built the exchange, and the final law also reflects that belief as well.”