Part II of the book shifts in focus from primary transformation to party transformation. This chapter tests whether primaries can change parties under the most likely conditions: Republican factional primaries in the Tea Party era. Using a difference-in-differences design, this chapter shows that in this most visible case, pressure from the reactionary Republican faction in primary elections served to reorient the party rightwards. It therefore demonstrates that, under the right circumstances, primaries can contribute to partisan polarization in Congress.