Despite widespread speculation among pundits and politicians, statistical research finds little evidence that primaries are an important source of polarization in roll call voting. This manuscript moves beyond roll call votes by testing the effects of ideological primary challenges on partisanship in bill co-sponsorship in Congress. Moreover, while extant research generally focuses on the one-to-one effects of primary challenges on the incumbents who experience a challenge, I measure and test the effects of the mere threat of a primary challenge from the ideological extreme. I find that the increased threat of an ideological primary challenge accounts for about one-fourth of the rise in partisanship that occurred from the 1980s to the 2010s. These findings suggest the recent wave of ideological primary challengers is an important source of the escalation and intensification of polarization in recent Congresses.