Over the past thirty years, two developments have altered the makeup of the U.S. Congress. The first and more widely recognized of these is the rise in partisan polarization. The second has, by contrast, gone largely unnoticed: the number of Democratic women in Congress has increased dramatically while the number of Republican women has barely grown. My dissertation develops a theory of party fit to explain both of these trends. First, I show that patterns of candidate entry contribute to partisan polarization in Congress. Candidate emergence has received little attention in the polarization literature, but I find that liberal Republican and conservative Democratic state legislators are less likely to run for Congress than those at the ideological poles. Second, because Republican women have historically been to the ideological left of their male counterparts, I suggest that the rightward shift of the GOP has had a negative effect on the representation of Republican women. Thus, the focus on party fit enhances our understanding of over-time changes in the ideological and gendered makeup of Congress.