Partisan polarization is undeniably the central and most problematic feature of contemporary American politic. Political parties today are more internally unified and ideologically distinctive than they have been in over a century. … The center of gravity within the Republican Party has shifted sharply to the right. … The Democratic Party … has hewed to the center-left, with an emphasis on the center, on issues ranging from welfare reform to health policy.
From pages 44 and 52 of It’s Even Worse than it Looks: How the American Constitutional System Collided with the New Politics of Extremism by Thomas Mann and Norm Ornstein. This is not merely an argument; there is ample empirical support for this proposition.
To me, this is arguably the most important concept that Centre College Government majors (and all American citizens, really) need to understand in order to be able to critically evaluate contemporary American politics.