It does not need to be a coordinated attack - USA alone is enough to scare investors, sure the anarchists will stay with bitcoin till the end - but USA can destroy the exchanges.
mtgox is in Japan. But lets say they shut them down, the volume would be back, distributed among the rest of the platforms within a month, prices in 2 months.
Even if they manage to shut down all the popular exchanges (btc-e is in russia btw, good luck with that) there would be new ones after a week.
They can stop american banks talk to them - this would be enough to send the price in a down spiral.
That said - I don't really believe that they would do it before bitcoin starts the real disruption - the government machine cannot outlaw something that easily, it takes years to come to a reasonable legislation etc. But if stuff like what Falkvinge wrote about
http://falkvinge.net/2011/05/19/the-information-policy-case-for-flat-tax-and-basic-income/ happens then we'll see what the US government can and what it cannot. If until that time bitcoin has a lot of business going on it and not just the drugs and rhino horns - then this will be a fight - otherwise they'll crush the currency by undermining trust in it. A currency without trust is worthless.