I think 51% can be done quite easy for the U.S. government: they need special squads/agents to take over of 2 or 3 biggest mining pools. This can be done even if pools are operated outside of the U.S., and even can be done remotely. But i guess, to have more confidence they would need to send special people to pool operators (-; Hell, at the moment we cannot be sure that some of the pools are already in control of governments....