The only way I can see the US capital moving is to have a CSA wins TL. Then the US almost has to move it's capital. DC is right on the border of Maryland and Virginia. It could get very uncomfortable if your capital were situated on the border of a foreign country. Otherwise I don't see the capital moving short of its destruction. If your goal is simply to have a US capital that is not DC, the best bet would be to stop it from being located there in the first place.
I still think it'd be kind of cool to have Philadelphia be capital, myself.