What about these answers (just in case you are looking for at least an attempt at an actual answer, and not just the humor offered up by Jim & Conor...which may have been your original point anyway, to embarrass them, and they fell into your trap)

External: Islamists; China

Internal: the National Debt.

I could list the communists taking over the Democrat Party, but that kind of falls within the National Debt problem, since they will just print money and spend it on wasteful/fradulent programs like there is no tomorrow. But, wow, think of the damage that could be done if they actually gain momentum. Hard to believe, but who knows.