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.”