Supporters of the Affordable Care Act gather in front of the U.S Supreme Court during a rally March 4, 2015 in Washington, D.C. (Alex Wong/Getty Images)

The Supreme Court heard oral arguments this week in King v. Burwell, a case with potentially dramatic consequences for the Affordable Care Act (ACA). This case has appropriately invited a great deal of discussion on how the courts tackle statutory interpretation. But we should also consider why they so often have to do so in the first place, especially on such high stakes issues. My research shows that the partisan polarization that has made the ACA so contentious is also a crucial factor in producing ambiguous statutes.

At issue in King v. Burwell is income-based eligibility for subsidies under federally run health-care exchanges, in addition to the state-run exchanges. The statute refers to exchanges “established by the state,” but Internal Revenue Service rules apply those subsidies to participants in the federal exchanges as well. Thirty-four states participate in the federal exchanges, and several million residents in those states receive subsidies. The court’s ruling will thus not only affect those who are currently eligible for the subsidies, it could also unravel the extended pool of coverage that serves as the underpinning of the ACA.

This case hinges on statutory interpretation – specifically of those oft quoted four words “established by the state”, and how those words fit into to a long, complex and sometimes ambiguous law. A number of thoughtful observers and have weighed in on how the court should or will interpret this phrase, invoking precedent, judicial philosophy, legislative intent, statutory reading, and the consequences of the court’s decision.

Challengers of the ACA promote a “literal” reading: the word “state” means only exchanges run by the state are eligible for the subsidy. Defenders of the law argue that this narrow interpretation is out of step with the clear intent of the legislation as a whole, which is predicated on subsidies being available in every state.

Why is the ACA embroiled in this battle? As I argue in my recent book, “The Devil is in the Details:Understanding the Causes of Policy Specificity and Ambiguity,” vague laws necessitate rules and regulations, and sometimes provoke litigation, to elaborate on their meaning and resolve inconsistent interpretations — in effect, to fill in the blanks. (See also these related studies.) More precisely worded policy provides more direction and less discretion to bureaucratic and judicial actors. Supreme Court justices themselves have complained about ambiguous statutes. In Ruth Bader Ginsburg’s words, “The national legislature expresses itself too often in commands that are unclear, imprecise or gap ridden”.

So why then, do some laws end up precise and others vague? My research examines health and welfare policy in the states. I find that ambiguous statutes tend to arise where political conditions are fragmented and contentious. Ambiguous statutes are more likely where there is partisan discord, where there are more and more diverse interest groups active in politics, and where the governor has less direct control over state agencies. Moreover, the effects of fragmentation are more pronounced for policies that are highly salient and receive more coverage in the press.

Put simply, on important and controversial legislation, ambiguity serves as a vehicle for compromise when key participants disagree over details.

One of the most significant factors in promoting statutory ambiguity is party polarization – the ideological distance between the two parties. Scholars and pundits frequently observe that party polarization leads to policy gridlock. It also leads to less precise policies. Of course, this may be good news, insofar as it makes compromise achievable and allows for necessary flexibility in implementation. At the same time, those compromises leave important policy debates to be resolved outside the legislative process, frequently by unelected actors.

While my book focuses on state policy-making, the lessons are clearly applicable to the ACA. The law is high profile and controversial, and it was crafted in an environment marked by polarized parties and diverse interest group activity. When the law passed in 2010, party polarization in Congress was nearing an all-time high. Hundreds of interest groups lobbied for and against the bill. Small wonder, then, that we continue to fight over the details.

As we await the decision in King v. Burwell, we should remember that the language of a law is a political story as well as a technical one, and that fights over the meaning of a law frequently continue well past its adoption.

Rachel VanSickle-Ward is an Associate Professor of Political Studies at Pitzer College, where she specializes in policy, law, and women in politics.