At times, I wonder how distinct actually the field of American politics (AP) to comparative politics (CP). Yes, the focus of analysis of the two is obviously different. AP, the name speaks for itself, focuses on political dynamics in the United States. One seeks to study AP, for example, will be socialized with the ways American democracy works. They can study congress or presidency, how gender and race affect political discourses, public opinion, or voting behavior. On the other hand, the focus of CP potentially intersects with the broad themes in AP. None of the subject of interests in AP are alien for CP. Instead, the topics of CP are quite broader (i.e very few political scientists have interest to study social movement or leftist ideology in the US, whereas these topics have been scrutinized in great details by comparativists). Having taken both CP and AP, I can tell that the considerable area of interests between the two are not that different.
But the distinction between the two subfields is more obvious if we compare the requirements for methods courses. To get you a sense, here are brief descriptions for AP and CP at the Ohio State, one of top-notch PhD programs in the US. As you can tell, there are minimum FIVE courses on methodology (from quantitative methods to formal model) that must be taken by those who study AP. Whereas in CP, the requirements are somewhat modest. Students are usually expected to take the combination of research design and quantitative class, aside from a foreign language requirement of the country of interest (which is, predominantly, has been eliminated in many schools).
So as you might expect, in the first place, I can say that the field of AP is mostly method-driven than CP, which is largely, I suppose, a problem-driven field. AP is methodologically more rigor and more quantitatively oriented field than CP (Ian Shapiro has an excellent discussion on this). This is not to say that such a methodologically oriented field is wrong. That’s not the point. But I was just wondering how isolated AP could be if scholars engage in AP debates don’t put their findings in comparative perspective. This idea also calls into question how compliment or how possible the scholars in the two subfield can communicate each other, given boundary in their respective fields (more discussion on this Linz & Stepan and Przeworski).