The U.S. Supreme Court returns for arguments on Jan. 12 in a term likely to be defined—as its terms were in 2013 and 2014—by a battle over President Barack Obama’s signature health insurance reform.

While many court watchers await some sign on whether the justices will consider same-sex marriage, there is little doubt that King v. Burwell, with its potential to seriously damage the Affordable Care Act’s coverage of millions of Americans, is the priority for supporters and opponents of the still controversial law.