Chapter 5 examines the driving forces behind polarization in greater detail, focusing on open seats and asymmetric polarization. Open seat candidates have a far greater probability of being elected than do challengers. Compared to the few candidates who defeat incumbents, open seat winners have the largest influence on the party’s ideological course. This chapter shows that across state legislators, the probability of seeking congressional office increases when the seat is open, though the likelihood of running soars among conservative Republicans and liberal Democrats. Chapter 5 also illuminates an additional mechanism for asymmetric polarization, as candidate entry on the Republican side differs from that on the Democratic side. I draw on both CFscores and Shor and McCarty’s (2011) estimates of state legislator ideology. I find that moderate Democratic and Republican state legislators have both opted out of congressional elections, but conservative Republicans are more likely to run for Congress than liberal Democrats. The findings are crucial for understanding the action of polarization and the specific forces that are driving asymmetric polarization.