Recent research asserts that coalitions of party leaders, interest groups, and activists will cooperate to support the nomination of mutually acceptable candidates in primary elections. In this article, I utilize an original dataset containing FEC contributions and expenditures data for 1,648 candidates who ran in open seat primary elections for the US House from 2006 to 2016 to measure the extent and effects of coordination among interest groups and party organizations. I find that Democratic-aligned interest groups and party leaders coordinate more often and with a more positive substantive effect than their Republican counterparts. Moreover, I provide evidence that, with the advent of super PACs in the second half of the 2010 primary cycle, a small number of interest groups can act as a latent threat to broader coalitions that unite behind a candidate using independent expenditures to outspend the broader coalitions. This increased resource parity has tangible representational consequences.