Politicizing the investigation of politically active groups is harmful for both the justice system and democratic accountability. I test whether members of the US Congress affect the investigation and prosecution of politically active labor unions. Union officers are 1.5 percentage points more likely to be prosecuted when their supported candidate barely loses instead of barely wins (compared to the 3% base rate). Anecdotal evidence and a novel decomposition suggest a role for both union-supported winners protecting allies and union-opposed winners pushing for aggressive prosecution of their opponents. I show prosecutions undermine unions' strength and calculate implications for the incumbency advantage.