"Poltical promise" eh? Nice euphemism for a lie.
A lie is an intentional misstatement of what has occurred. If I say I am going to give you $20 Monday and don't, I have not lied. I may be dead and can't give you the money. You may be dead. I may forget. A host of reasons--some good, some not so good--why people don't do what they say they will. In this case Bush hasn't even done that. He said he would sign the bill and apparently he will. That he thinks it will never get to his desk does not make him a liar.
If I say I already gave you $20 and know I didn't--that is a lie.
This is not hairsplitting. Politics would be a lot better if language was used correctly.