A congress that banned BitCoin would have to be sure that such a ban did not violate The Constitution, in particular the First Amendment.
A Federal ban would probably instigate legal cases against the ban, putting the value in limbo, especially if there are injunctions. It may pit the Executive branch against the Legal branch given the communication aspect of BitCoin transactions clearly make it a form of expression.
Ahahahaha this made my night. As if the US Congress respected the Constitution! The very dollars they now print are unconstitutional, and such hangups as "the Constitution" will never prevent them from assaulting Bitcoin, unfortunately.
The backing by gold/silver was removed from US dollars in phases over a long period of time.
If they do try to outlaw bitcoin, it'll probably done in multiple steps.