This article focuses on how party identity can shape policy support or opposition to the controversial issue of legalizing cannabis in Sweden, which is strongly opposed by the public. In a survey experiment (N = 3612), we manipulated if a message that supported or opposed a policy proposal to legalize cannabis was presented by a representative of the own party or an outgroup party. Results showed increased opposition to the proposal when the ingroup party opposed the policy and when the outgroup party endorsed the policy. When the ingroup party endorsed the policy and when the outgroup party opposed the policy, attitudes to the policy were not influenced. We argue that prior attitudes moderate how ingroup- and outgroup party messages are processed and that voters do not blindly follow the party line. Only when the own party presents a position that coincides with the individual’s prior position, are attitudes strengthened and voters follow the party line. Attitudes are also strengthened as a way to increase distance to a disliked outgroup party. When the party cue contradicts prior beliefs (ingroup-endorse; outgroup-oppose), the information is ignored, which allows individuals to retain their view of the party, be it positive or negative.