How those words became the most contentious part of President Obama’s signature domestic accomplishment has been a mystery. Who wrote them, and why? Were they really intended, as the plaintiffs in King v. Burwell claim, to make the tax subsidies in the law available only in states that established their own health insurance marketplaces, and not in the three dozen states with federal exchanges?

The answer, from interviews with more than two dozen Democrats and Republicans involved in writing the law, is that the words were a product of shifting politics and a sloppy merging of different versions. Some described the words as “inadvertent,” “inartful” or “a drafting error.” But none supported the contention of the plaintiffs, who are from Virginia…

“I don’t ever recall any distinction between federal and state exchanges in terms of the availability of subsidies,” said Olympia J. Snowe, a former Republican senator from Maine who helped write the Finance Committee version of the bill…

“As far as I know, it escaped everyone’s attention, or it would have been deleted, because it clearly contradicted the main purpose of the legislation,” Mr. Bingaman said. He added, “In all the discussion in the committees and on the floor, I didn’t ever hear anybody suggest that this kind of distinction between federal and state exchanges was in the bill.”