The USG, as powerful and full of unlimited hubris as they are, cannot simply make bitcoin illegal carte blanche, although I'm sure many of them would like to do that if they could get away with it. If they were to attempt to do so, they risk looking completely toothless when everything they try is ineffectual. What will more likely happen is (1) the constant demonization of bitcoin as for money laundering, drugs, child porn, kooky libertarians, or just being a big pyramid scheme, ponzi scheme, worthless digital fantasy, blah blah blah, we've seen all that already, and I expect these attacks to only intensify. Basically they are trying to scare the not so tech savvy older folks away from BTC. They want to make it seem sketchy and illegal-ISH to discourage widespread adoption. (2) They could try to introduce highly restrictive regulations on merchants being able to accept bitcoin as payment. This would stifle adoption of bitcoin, but would not stop it. The potential advantages of BTC over costly credit card fees, etc., is incentive enough for market adoption. Their best bet is to utilize shady operatives to DDOS the main exchanges, or attack the network in other ways, and I imagine that will (or maybe already has) achieved some success. But the BTC network grows stronger by the day, and eventually much better exchanges will evolve.

I think the USG will find ways to stifle bitcoin development, but in the end they will be incapable of stopping it, even if it threatens to topple the banksters. Never underestimate the power of the market. Remember, the USG, as with all other governments, cannot rule by fear and force alone. This is why they labor tirelessly to propagandize the masses, they utterly depend on willful consent to some extent. If the market decides it loves bitcoin and opts to adopt it full scale, there will be absolutely nothing any government can do to stop it. All they can do at that point is delay the inevitable.