You may have noticed that our district vote and support plot seemed to indicate that the major component of our line was reflective of the difference between two distinct sets of Congress members. In fact, the two clusters represent Republican members on the lower left, and Democratic members on the upper right. What we have is a type of spurious relationship, where a third variable is the true independent variable that causes both the district vote for Obama and the district member’s subsequent support for him. Districts that vote Democratic for president will usually vote Democratic for Congress. These Democratic members will usually share many of the policy perspectives of a president of their party. Thus, it is the partisan nature of a district that influences both the presidential vote and, less directly, the support that district’s representative affords the president (see Figure 12.1).