The aftermath of a national tragedy generally elicits two responses. The first is the expression of collective grief, a crucial element of the healing process. The second is the question, quintessentially if not uniquely American, of what can be done to prevent such events.

Katrina launched a debate about hurricane preparedness, the Sept. 11 attacks a broad inquiry into antiterrorist efforts. Only for one category of tragedy, mass shootings, is the obvious set of questions -- What went wrong? What should be changed? -- so emphatically off the table, at least for politicians.

The reality of modern politics, which is to say the political firepower wielded by the National Rifle Association, stifles any serious discussion of gun control. That has been the arc, as predictable as it is disappointing, of political responses to last week's massacre in Aurora, Colo., most notably from President Obama and Republican presidential candidate Mitt Romney.

The closest the president edged to the topic was on Sunday, when he expressed hope that "we all reflect on how we can do something about some of the senseless violence that ends up marring this country."

Such as? White House press secretary Jay Carney semi-elaborated: "The president's view is that we can take steps to keep guns out of the hands of people who should not have them under existing law," he told reporters. But there is no indication that existing law would have done anything to stop James Holmes, the suspected gunman.

Romney was similarly heavy on soothing rhetoric and light on specifics. "Today, we not only feel a sense of grief but perhaps also of helplessness," he said after the shooting. "But there is something we can do. We can offer comfort to someone near us who is suffering or heavy-laden. And we can mourn with those who mourn in Colorado."

New York Mayor Michael R. Bloomberg was right to call out both candidates on such inadequate responses. "Soothing words are nice," he said on Friday. "But maybe it's time that the two people who want to be president of the United States stand up and tell us what they're going do about it."

No gun-control law, however restrictive, can reliably stop every massacre. But in the aftermath of Aurora, Tucson and Virginia Tech, politicians should not be permitted to evade a serious discussion of whether additional restrictions are warranted.

The assault-weapons ban that expired in 2004, although riddled with loopholes, would have covered the AR-15-style rifle used in the shooting. During the 2008 campaign, Obama supported renewing the ban, but he has since been silent on the subject. Romney once supported the ban as well, but he has renounced that view.

The assault-weapons ban also prohibited the manufacture of magazines of more than 10 rounds. Holmes, according to police, bought a high-capacity "drum magazine" that could hold 100 rounds and fire as many as 60 rounds in a minute. High-capacity magazines were also used in the Tucson and Fort Hood shootings. Do the presidential candidates support renewing the limit on magazine sizes?

Holmes allegedly placed online orders for an astonishing 3,000 rounds of handgun ammunition, 3,000 rounds for an assault rifle and 350 shotgun shells. Should online sales of ammunition be prohibited or more carefully scrutinized? The failure of the presidential candidates even to broach these issues is a disservice to the Americans they hope to lead, and to the memory of the victims of Aurora.