In March 1997, two congressmen proposed a welfare law so detailed that it had 77 sections. Within a month it sailed through the House by voice vote.

There was nothing unusual about swift passage for the Welfare Reform Technical Corrections Act of 1997. But in retrospect, it underscores the deteriorating conditions in Washington today, as the Supreme Court has accepted a case threatening the Affordable Care Act over the interpretation of a single ambiguous phrase.

The 1997 act passed a year after President Bill Clinton and a Republican-led Congress had struck a broader deal on welfare, a racially charged issue that had roiled politics for two decades. The passage of the 1996 law followed fierce fighting — Mr. Clinton vetoed two welfare bills before signing the third — as well as a bitter budget fight that had temporarily shut down the government the previous winter.

Frustration over the consequences underlies Mr. Obama’s determination to alter the immigration system with a stroke of his pen — which in turn promises to exacerbate those divisions.

Ms. Wilensky holds out hope that the court might yet prod Congress to act. If a majority of justice’s call on lawmakers to resolve the ambiguity, she reasoned, Republicans might then salvage the challenged subsidies in return for changes such as curbing health-care mandates on employers.

“People of good will on both sides got together, they didn’t always agree, they worked out their differences and they dealt with problems,” Mr. Aaron said. “That is all too regrettably now missing from the scene here.”