Guns. There is currently no more emotional issue in the US and nothing that polarizes more. You’re either in favor of guns or you aren’t, and if you’re in favor of them the ownership of a gun is probably viewed as a fundamental right. There isn’t much room for compromise.
Into this debate we have a nearly constant litany of shootings, sometimes with legal weapons and sometimes not. President Obama has decided to act where he can, more or less working on tightening up the existing background check laws and enforcing them more uniformly and rigorously.
There was a time when gun advocates called for us to “enforce existing laws” rather than write new ones, but even this action is controversial. But it shouldn’t be.