Perhaps the most important moment in Wednesday's Senate Judiciary Committee hearing on guns came when retired astronaut Mark Kelly directly confronted the National Rifle Association's Wayne LaPierre over the shooting of Kelly's wife, Gabrielle Giffords. LaPierre had repeatedly voiced the talking point that there is no need to expand the background-check system because criminals don't cooperate with background checks.Kelly responded: "The Tucson shooter was an admitted drug user. ... He was clearly mentally ill. And when he purchased that gun in November, his plan was to assassinate my wife and commit mass murder at that Safeway in Tucson. He was a criminal. ... He would have failed that background check. He would have likely gone to a gun show, or a private seller, and avoided that background check."Requiring private sellers to complete background checks and getting those records into the system, Kelly said, "will prevent gun crime."There are two policy conundrums here. First, how do you ensure that people such as the Tucson shooter are represented in a database designed to prevent criminals and the mentally ill from getting guns? The database has numerous gaps. But some of these would be addressed by the president's proposals.Second, how do you expand the background-check system so it screens more gun sales? The Obama proposal would do this by closing the loophole that allows guns to be sold without background checks at gun shows and by private sellers.There is no evidence for LaPierre's suggestion that background checks don't work. As my Post colleague Glenn Kessler has shown, in 2010, alone checks denied guns to tens of thousands of people with felony and criminal backgrounds. More broadly, background checks have blocked more than 1.5 million gun sales to people who are prohibited from having guns.There is no way to know what would have been done with those guns had those sales gone through. But the question for those who oppose background checks is simple: Do you think the country would be better off if those sales had gone through?If criminals don't cooperate with background checks, or get guns from private sellers or gun shows, or from gun dealers who get them via such means, that is an argument for expanding the background-check system -- not an argument against it. These loopholes are the reason gun crime persists even in areas that have strict gun control.The "gun rights" crowd likes to cite Chicago as an example. But as Illinois Sen. Dick Durbin, a Democrat, said Wednesday: "When you take a look at where these guns come from, 25 percent-plus are sold in the surrounding towns around the city of Chicago, not in the city. Look over the last 10 or 12 years. Of the 50,000 guns confiscated in crimes, almost one out of 10 crime guns in Chicago came to that city from Mississippi. Why? Because the background checks there, the gun dealers there, are a lot easier than in other places. And they end up selling these guns in volume."Baltimore County Police Chief Jim Johnson, who chairs the National Law Enforcement Partnership to Prevent Gun Violence, echoed the arguments of Kelly and Durbin almost to the letter. That means the NRA, the gun industry and leading GOP (and a few red-state Democratic) officials are on one side and law enforcement officials and the vast majority of the American public -- Republicans, NRA households and gun owners included -- are on the other.Greg Sargent writes The Plum Line blog for The Washington Post.