I think that an interesting argument here could be that American political parties are not actually that different. The Republicans and the Democrats behave in surprisingly similar ways. This is what makes them unique, as many voters believe that there is a difference between them. The reality is that there are many ideological and behavioral similarities. In terms of political behavior, they behave strikingly similar to one another. Both are usually zealous in support of a President from their party and strong advocates for any political leadership that is in their name. By the same toke, both parties are unnecessarily harsh towards the oppositional party. The behavior of partisanship is strikingly similar on both sides of the aisle, clearly indicting that a unique feature of both political parties is their similar behavior.
Ideologically, neither Republican or Democrat favors a massive shift in who owns the means of production or in economic reality. Both are in favor of capitalism and are rather unapologetic about it. Given this reality, both political parties approved the government bailout, an extraordinary measure of external intervention. Both sides are unapologetic about "the War on Terror" and ensuring that "the bad guys" are apprehended and pursued. Republicans and Democrats both seek to circumvent campaign financing regulations and find ways to generate more money in their elections. With all of this in mind, the most unique aspect of both dominant American political party is how similar they are to one another.