Previous research has documented that the institutional behaviors (e.g., lobbying, campaign contributions) of political organizations reflect the polarization of these organizations along party lines. However, little is known about how these groups are connected at the level of individual party activists. Using data from a survey of 738 delegates at the 2008 Democratic and Republican national conventions, we use network regression analysis to demonstrate that co-membership networks of national party convention delegates are highly polarized by party, even after controlling for homophily due to ideology, sex/gender, race/ethnicity, age, educational attainment, income, and religious participation. Among delegates belonging to the same organization, only 1.78% of these co-memberships between delegates crossed party lines, and only 2.74% of the ties between organizations sharing common delegates were bipartisan in nature. We argue that segregation of organizational ties on the basis of party adds to the difficulty of finding common political ground between the parties.