What is a party? We argue that the formal party apparatus is only one part of an extended network of interest groups, media, 527s, and candidates. We systematically measure a portion of this network by tracking transfers of names between political organizations. Our analysis reveals two distinct and polarized networks corresponding to a more liberal Democratic group and a more conservative Republican group. Formal party organizations, like the Democratic National Committee and the Republican National Committee, tend to receive information within their respective networks, which suggests that other groups serve to funnel information toward the formal party.