Although a rich body of research has explored the sources of party polarization in the US House of Representatives, it has focused primarily on the House since the late 1970s. Drawing on a dataset of historical election outcomes, legislative voting and survey data, this article takes an alternative approach that examines both the US Senate and the House in their broader historical contexts. It is argued that the unusually bipartisan era of the 1950s created a set of circumstances that enabled congressional parties to remain relatively unpolarized throughout the 1960s and early 1970s. Although the national parties became more ideologically distinct in the mid-1960s, congressional parties lagged behind. As a result, a group of moderate legislators emerged who were cross-pressured between their national parties and their constituencies. Only when natural patterns of electoral loss and retirement replaced these legislators did congressional party polarization re-emerge.