Intra-party conflict has become increasingly visible in national U.S. politics, especially within the Republican Party. Despite this turmoil, the Republican Party remains asymmetrically polarized. We hypothesize that factional pressure from a far-right faction (the Tea Party) has exacerbated the party’s trends towards mass and elite polarization. We test this hypothesis through a granular analysis of Republican factionalism at the congressional district level. We develop a measure of local factionalism using novel datasets of factional activist presence and primary challenges. We implement a difference-in-differences design to assess whether local factionalism in the Tea Party period (2010—2014) heightened Republican partisanship and legislative extremism at the district level between 2008 (pre-treatment) and 2016 (post-treatment). We find that factional pressure did force congressional districts rightward on both measures. These findings clarify how the Tea Party captured the Republican Party and support a focus on the role of party factions in fomenting partisan conflict.