This ad is basically just bigotry towards atheists. Everybody has a right to voice their opinion including atheists.
Basically it infers that associating with atheists makes you an atheist yourself.
Maybe "Godless Americans" agree with Kagan on issues other than God, you know like good government and not smearing your opponents based on the religious beliefs (or lack thereof) of a small minority of their supporters. Beliefs that those supporters have EVERY right to have.