Dick Heller, a longtime resident of the District of Columbia, carries a handgun for his job as a private security guard. But at the end of his shift, he packs up the .38 revolver and stashes it in a vault. He would like to keep a gun for protection at his Capitol Hill home, where he has endured the sound of gunfire for years. But he can't, because D.C. law forbids it. "They give me a gun to protect them," he says of the government, "but I'm a second-class citizen when I finish work."

One of the most restrictive gun laws in the country, the D.C. statute is the focus of a March 18 U.S. Supreme Court hearing that marks the most significant case on gun control in decades. With Heller as plaintiff, it is the first test since 1939 of whether the Second Amendment supports an individual's right to bear arms and not just a state's right to form a militia. It is a crucial distinction. A ruling in favor of the individual right could trigger a wave of constitutional challenges to gun

control laws nationwide. And it could suddenly bring a volatile issue—one particularly uncomfortable for Democrats—into play during a presidential election year.

"It's significant because either it's going to fuel attempts to restrict gun ownership or it could put a constitutional wet blanket on any effort to control gun ownership," says Martin Redish, a constitutional law professor at Northwestern University.

For all the passion on both sides of the Second Amendment debate, the Supreme Court has said remarkably little over the years about to whom the right applies. Specifically, the amendment states that "a well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed."

For most of American history, courts have interpreted the Second Amendment to apply to the collective right of states to assemble groups of armed citizens, such as the National Guard. Nine federal circuit courts have upheld that position, and the Supreme Court favored it when it last considered the issue in the 1939 case. (While that decision upheld the federal regulation of an individual's use of sawed-off shotguns, it didn't directly address the scope of the Second Amendment.)