With the ease of downloading a song, anyone with a computer and a credit card can order thousands of bullets and shotgun shells on the Internet, along with tear-gas canisters and speed loaders. They can get the same high-capacity ammunition clips that infantry soldiers use. They can even get bulletproof vests and SWAT helmets. All without fear of a single background check.

No one is paying attention to whether buyers have criminal histories or mental-health records. No one is monitoring bulk sales of ammunition to see who might be building an arsenal. Even after a young man in Colorado buys 6,000 rounds by mail order and uses them to commit mass murder, it is the rare politician who proposes to make the tools of terror slightly harder to obtain.

When he was campaigning for office in 2008, Barack Obama vowed to reinstate the assault weapons ban that had expired in 2004. That would have prohibited the AR-15 rifle used in the Colorado theater shooting on Friday, along with the large 100-round magazine attached to it. But as president, Mr. Obama has made no attempt to do so. Mitt Romney banned assault weapons as governor of Massachusetts and undoubtedly saved many lives, but now he opposes all gun control measures. He never repeats what he said in 2004 when he signed the ban:

“Deadly assault weapons have no place in Massachusetts,” he said. “They are instruments of destruction with the sole purpose of hunting down and killing people.”

Both men fear the power of gun ideologues, particularly in swing states like Pennsylvania, Nevada and North Carolina, where many voters have fallen under the spell of a gun lobby that considers any restriction an unthinkable assault on the Constitution. Senator Ron Johnson, the Tea Party favorite from Wisconsin, spoke for the Republican Party (and many Democrats) when he said that limiting high-capacity magazines would infringe on a basic right. “When you try and do it, you restrict our freedom,” he said on “Fox News Sunday.”

Freedom to do what, precisely? To fire off 100 rounds without reloading? A few sport shooters may enjoy doing that on a firing range, but that’s hardly sufficient reason to empower someone else to do it in a movie theater. It has nothing to do with the basic right of home protection and self-defense found by the Supreme Court in 2008.

A Democratic senator, Frank Lautenberg of New Jersey, is one of the few officials courageous enough to propose a better idea: A ban on clips that hold more than 10 bullets, which are not needed to hunt, practice or protect oneself. He first proposed this last year, after a gunman in Tucson used a 33-round magazine to shoot 18 people, including Representative Gabrielle Giffords, killing six. The shooter was tackled when he had to reload.

The ban went nowhere and will undoubtedly be laughed off by gun advocates this year, too. In 1993, they killed a proposal by Senator Daniel Patrick Moynihan of New York to impose a heavy tax on handgun ammunition, especially the bullets that expand and cause heavy tissue damage. A few years ago, Gov. Arnold Schwarzenegger of California signed a law requiring identification to buy handgun ammunition and forbidding mail-order sales. A group of gun sellers sued and won a trial-court ruling that the law was too vague. (The state attorney general, Kamala Harris, appealed the ruling in February.)

But the gun lobby’s legal and political victories can’t obscure the facts. The assault weapons ban didn’t clearly reduce crime, the best study of the measure found, but allowing high-capacity magazines would “result in more shots fired, more persons hit, and more wounds inflicted per victim than do attacks with other firearms.” Sensible restrictions on ammunition and clips won’t eliminate mass shootings; they may make them less likely and reduce their level of violence.

Many politicians of both parties know this. To overcome their fear of the gun lobby, they need only look at the faces of the victims in Aurora, Colo.

6,000 Bullets