Mayors, Guns & Money: N.Y.'s Michael Bloomberg has picked a fight with the gun lobby. His arsenal: 450 other mayors and his own deep pockets.

largest single financial backer of Mayors Against Illegal Guns, but the mayors also talked the real estate magnate Eli Broad into kicking in $750,000 and the Chicago-based Joyce Foundation into contributing $1.1 million. 'If you believe in something,...