Much as I agree with the sentiment behind it, I find this ad by the Center fof Constitutional Rights rather unpersuasive:
That said, however, Fox News's explanation for why they won't air it is nothing short of hilarious:
We cannot approve the spot with it being Danny Glover's opinion that the Bush Administration is destroying the Constitution. If you have documentation that it is indeed being destroyed, we can look at that.
I understand that Fox even sent an intern over to the National Archives and he said it looked like the Constitution was in pretty good shape and it was under glass and everything so the president probably couldn't even get at it if he wanted to. (Via Andrew)