Granted, expanded background checks are only one element of what's needed to stem gun violence. But Jonathan Lowy, director of the Legal Action Project of the Brady Center to Prevent Gun Violence, told me it was, by far, the most important element. Yes, bans on assault weapons and high-capacity magazines are also critical. So is a crackdown on straw purchases. But if we did only one thing this year, said Mr. Lowy, universal background checks are the key. For a very simple reason. Because, under existing law, criminal background checks are only required for gun sales at licensed gun dealers. In a Jan. 16 speech on gun violence, President Barack Obama claimed that "as many as 40 percent of all gun purchases are conducted without a background check." Which means that at gun shows, on the Internet, from classified newspaper ads or over the back fence, purchases are made with no prior check on the buyer.