There can be many ways to go about answering this. I would take the economic materialist point of view. For the most part, both parties have not represented a true departure from the capitalist practices that have helped to define America in the last 150 years. I think that this is part of the reason why they have endured. In the end, while America strives for different expressions of the political notion of the good, industrialization and commerce have helped to make its economic articulation a capitalist and free enterprise one. The Democratic and Republican parties have, for the most part, represented an embrace of these ends. There have been moments where this has not been as true, but for the most part, one can say that both parties have done their part to support free enterprise and capitalism as the primary means of economic exploration of the good in the nation. This has been certainly the case over the last two decades, and has helped to explain why the Democratic and Republican parties, despite political differences, end up sitting at the same economic table of profit and wealth.