The question becomes who decides if a law is repugnant to the Constitution? Marshall answered that as well - the courts, not the president or the sheriff. So if Obama's gun control legislation is ruled unconstitional by the courts then the law enforcement agencies cannot enforce them, no matter what Obama says. But before then...?
And President Andrew Jackson answered Justice Marshall regarding Worcester v. Georgia "...the decision of the Supreme Court has fell still born, and they find that they cannot coerce Georgia to yield to its mandate," (that is, the Court's opinion because it had no power to enforce its edict) (which was popularized as "John Marshall has made his decision; now let him enforce it!"
Neither Obama nor the Supremes have a mechanism for removing a sheriff for refusing to expend resource to enforce a federal law he disagrees with.
The Administration's henchmen and village idiots seem to be confounding SHERIFF with MARSHALL and STATE with TERRITORY.
In many instances you'll have state and federal criminal statutes that correspond, and there'll be some agreement ~ usually made as a consequence of law ~ to cooperate so if a local government picks up a criminal for a crime which is also a federal crime, they can hand him over to the federales, and vice versa. Some crimes are easier to deal with locally than others, and vice versa. Prosecutors of the various affected jurisdiction will get together over the bag of heads and see who wants to deal with that one!