You see a cross in that ad? Strange. Which of the three Roman crosses at this link does it look like to you?
http://www.christiananswers.net/q-abr/abr-a013.html
To me, it looks like a bookshelf. Whoever designed the set might have been trying to give the impression of a symbolic cross, but the actual image is not a Roman cross.
In any event, none of what you’ve written provides any proof that the Huckster knew that was the design purpose. I’m not in favor of his candidacy, but this line of attack is petty and silly.
Whether it’s a bookshelf as you think, or a window pane as I thought, I agree that there’s hardly conclusive proof that this was some kind of subliminal message to Christians. There has been nothing subliminal about Huckabee presenting himself as a Christian candidate, or even “the” Christian candidate, so I see no reason for him to enage in some kind of tricky subterfuge to make that point.