On the 11th anniversary of the Columbine shootings, Mayor Bloomberg’s Mayors Against Illegal Guns has launched a new campaign to pressure Congress to close the so-called “gun show loophole” that allows firearms to be sold without background checks.

The campaign includes an on-line petition drive and a series of ads that reference the Colorado high school massacre and also note that both President Obama and his GOP opponent, Sen. John McCain, pledged to close the loophole during the 2008 campaign.

The ads target both Democrats and Republicans, including: Sen. Scott Brown (MA), Sen. George Voinovich (OH), Sens. Olympia Snowe and Susan Collins (ME), Sen. Mark Udall (CO), and Sens. Mark Warner and Jim Webb (VA).

Bloomberg appeared on CNN’s “American Morning” to discuss the new effort, saying:

“They deliberately left this out and this is the way the Columbine massacre murderer got the guns. This is the way cops on our streets are getting shot. A third of the illegal guns that we find when we arrest somebody for committing a crime comes from gun shows. You can trace them back.”

“And it’s just got to stop. It’s killing our children, it’s killing our police officers…this is nothing to do with your right to bear arms. Every state has the right to enact their own regulations…You can do this at the state level, but it’s so easy to carry a gun from one state to another.”