President Obama is in a fix over firearms. He needs to win undecided voters in the swing states to be re-elected, but these areas are largely pro-gun. So after years of trying to dodge the issue, Mr. Obama let it slip in Tuesday’s presidential debate that he’d push a gun ban in a second term. It’s a revelation that could sway the election. In the town-hall event at Hofstra University, “undecided” voter Nina Gonzalez asked the president what he’d done to limit the availability of assault weapons. Mr. Obama feigned support for the Second Amendment before calling for regulation of...