Conventional accounts of Donald Trump’s unexpected electoral victory stress idiosyncratic events and media celebrity because most observers assume this unusual candidate won without much organized support. However, considerable evidence suggests that the support of conservative organizational networks, including police unions such as the Fraternal Order of Police (FOP), propelled Trump to victory. The FOP is both a public-sector union and a conservative, mass-membership fraternal association that was courted by the Trump campaign at a time of politically charged debates about policing. Four years before, the FOP had refused to endorse Republican candidate Mitt Romney because he opposed public-sector unionism, which provided fruitful and rare variation in interest-group behavior across electoral cycles. Using a difference-in-differences approach, I find that FOP lodge density contributed to a significant swing in vote share from Romney to Trump. Moreover, survey evidence indicates that police officers reported increased political engagement in 2016 versus 2012. Belying the notion that Trump lacked a “ground game,” this research suggests that he tapped into existing organizational networks, showing their enduring importance in electoral politics.