In the early part of the XIX century, American politics had a local flavor. Weak parties proved incapable of articulating national political identities and Congress operated in large part reactively to the given issues of the day. Yet despite the political fragmentation that characterized this period, by 1830 the contours of a national political stage had emerged (Formisano 1983). In this paper, we focus on the role of shared Congressional living arrangements as a cause of this ideological consolidation. We show that it was only when Congressmen from the South (North) lived in boardinghouses with other Congressmen from the South (North) that they realized their commonality of interests. Further, we use a particular aspect of our data—Congressmen that moved between boardinghouses at the end of the first session—to separate the impact of selection from that of political influence. Rather than choosing to live together on the basis of common regional interests, Congressmen recognized these interests because they lived together.