Well, if you read Atlas Shrugged you have a vision of how Right-wingers think America will be destroyed. Excessive regulation to the point of crippling entrepreneurship, limiting freedom of investment, freedom of operation, and freedom of movement. Paying people according to what they need not what they earn. Many of the things the USSR trotted out in communism, distorted, magnified and accelerated. Of course this won't happen because, not only does no one in government believe in outright communism, but if they actually did that even the vast majority of the Left (except the bunch they probably kicked out of the Democratic Party a long time ago) would resist it as well.
There is another way that could result in destruction, but its equally fantastic. The future reforms in the next 4 years create such a massive, inescapable fiscal hole, that the government is straddled with so much debt they must prioritize interest obligations before anything else. Hyperinflation would begin to manifest itself, OR the US would enter a period of severe stagnation ala Japan. And then to top it all off a foreign force invades the USA and chips off its land, and the military, crippled by lack of money to do anything, is powerless to fight back.
Again, that requires such a stretch of the imagination its impossible in today's world. The USA's size is, in many ways, its own protection; in order for any of the above scenarios to happen they must happen over many decades of slow, consistent decline. The US military is the largest in the world, its debt obligations by GDP is by no means the largest yet (aforementioned Japan takes the cake). The USA changing its politics every decade or so is also the key to its longevity; many of the fallen empires prior are marked with extended periods of one-party rule during the decline, and then destroyed (or renewed if lucky) in periods of internal infighting or civil war.