Mayor Bloomberg’s national antigun group is using yesterday’s anniversary of the Columbine shooting to launch a new campaign to close the so-called gun-show loophole.

Mayors Against Illegal Guns began running television ads yesterday in five states to appeal to US senators pivotal to passing legislation.

The local ads appeal to specific US senators who could be key in passing legislation. It would halt the practice of permitting unlicensed dealers to sell weapons at gun shows without conducting background checks.

Those senators are Mark Udall in Colorado, Olympia Snowe and Susan Collins in Maine, Scott Brown in Massachusetts, George Voinovich in Ohio, and Jim Webb and Mark Warner in Virginia.