In this paper, we analyze the roll-call voting behavior of House and Senate members who changed party affiliation during the course of their political careers. We analyze members who switched during the stable periods of the three major two-party systems in American history: the Federalist-Jeffersonian Republican system (3d to 12th Congresses), the Democratic-Whig System (20th to 30th Congresses), and the Democratic-Republican System (46th to 106th Congresses). Our primary findings are that the biggest changes in the roll-call voting behavior of party defectors can be observed during periods of high ideological polarization and that party defections during the past 30 years are distinct from switches in other eras because of high polarization and the disappearance of a second dimension of ideological conflict.