(CNN) One San Antonio police officer repeatedly used the n-word as he arrested a Black man for trespassing at a mall. Now, as protests over the police killing of George Floyd have refocused national attention on police misconduct, unions representing officers are facing a wave of new scrutiny and an unprecedented political backlash. How union contracts block accountabilityIn cities around the country, police officers are some of the most difficult government employees to fire thanks to favorable contract provisions negotiated by local unions. In several cities, officers have elected more bombastic leaders in recent years, thrusting their unions into culture war battles and strongly opposing reforms. San Antonio's contract with the San Antonio Police Officers Association goes farther, limiting past incidents arbitrators can take into account when deciding whether to uphold punishments.