All the evidence currently available indicates that the gunman responsible for Saturday's tragedy in Tucson, Ariz., was driven solely by internal demons. That fact hasn't stopped commentators, overwhelmingly on the left, from suggesting that today's "heated political rhetoric" is at least partly to blame.

Pundits have frequently cited Sarah Palin's "crosshairs map," which uses the riflery image to mark the congressional districts of vulnerable Democrats, as inspiration for...