Banning the exchanges is about all it would take. Besides, the current chorus is that large companies moving a share of their cash holdings into Bitcoin will drive the price way up. If the government bans Bitcoin then that's not going to happen.
Most probably a US ban will crash the price, but it will recover in time. Also it will establish a black market in the US which is the worst possible scenario for the authorities as they would have absolutely no control over it.
There already is a black market in for Bitcoin in the United States, that's basically all Bitcoin is used for these days aside from speculation. The fact that it would become a black market for Bitcoin itself doesn't change much. Almost all Bitcoin purchases these days are done through exchanges and held on exchanges, managing Bitcoin wallets is enough of a technical challenge that it will never be in any way widespread without the support of exchanges.