I agree that it would have to be something even more major than what is currently going on to incite a violent overthrow of the government. The actually asked if you have attempted or supported the 'violent' ovrthrow of the government on the security clearance questionaire... So I don't support a French revolution style overthrow because it will never work. The_Jazz is right, the FBI, local cops, state cops, national guard and regular military would be able to take out anybody causing trouble. They could get the media to not report about it, or spin it like they always do. This is sounding more and more like this event if anybody tries this.
http://www.cnd.org/HYPLAN/yawei/june4th/
(Banned in China)
What I would worry about is an attack on the rich & religious from the poor. Basically, the main Republican voters are the rich white guys and the extremely religious ones. So, if we have a civil war, that would be one way to eliminate enough of the voters of the opposite party so your guy wins in the next election.
I would have to trust that even a very Republican Congress (Or Democratic one if the Dems have the Presidency), would be strong enough to oust the President if they do something really bad. If for instance Bush called for the imprisonation and execution of all Muslims in the US and abroad. There would be major protests, but they would get arrested too. I don't know what actions the general populance could take against this government besides asking for help from a foreign country. Or moving to one.