I'm an actor of 20 years with both film and television in my background; I have a solid understanding of what happens during a shoot and in post-production. Two possibilities here:
The production team hired to film a national commercial for a leading presidential candidate cleverly lit part of a bookshelf brighter than the Christmas tree or even Huckabee himself to in order to not-so-subtly bring a cross into the image to reflect the message of God Bless and Merry Christmas, and everyone is lying about it.
The production team hired to film a national commercial for a leading presidential candidate are talentless, legally-blind hacks who errantly lit a bookshelf and, according to the Huck's story, not only didn't notice a blaring cross during the shoot, but didn't notice it in post-production, and only oh-my-gosh'ed when the public saw it.
I don't believe for a second that it's the latter.