>he shall have power to grant reprieves and pardons for offenses against the United States, except in cases of impeachment.
Is there some case law that has determined "offenses against the United States" == Federal crime only?
EDIT: And the root of this phrase seems to be in the distinction between civil and criminal offenses - an crime is an "offense against the State" [2], where the government has standing to bring the case to court rather than the individuals (if any) specifically harmed. In this context, the usage "offense against the United States" pretty clearly indicates crimes where the United States rather than an individual states has jurisdiction.
[1] https://en.wikipedia.org/wiki/Crimes_Act_of_1790
[2] http://criminal.findlaw.com/criminal-law-basics/the-differen...
At least going by the spiritual intent of the Constitution, the United States wasn't so much of a country on its own as a coalition of multiple tiny countries (made it easier for trade, war, debts), much as the member states of the European Union are still treated as independent countries today. The European Union hasn't yet had a 'civil' war to change the public opinion and perception of its governance, though.
The President doesn't have the authority to pardon someone for a State crime. I suppose he could threaten to pardon federal inmates who were residents of non-compliant states before their incarceration.
Article II, Section 2. The President…shall have the power to grant reprieves and pardons for offenses against the United States, except in cases of impeachment.
Without the abbreviation: The President shall be commander in chief of the Army and Navy of the United States, and of the militia of the several states, when called into the actual service of the United States; he may require the opinion, in writing, of the principal officer in each of the executive departments, upon any subject relating to the duties of their respective offices, and he shall have power to grant reprieves and pardons for offenses against the United States, except in cases of impeachment.