Republican Texas Sen. Ted Cruz and Democratic Chicago Mayor Lori Lightfoot shared a heated exchange on Twitter over gun violence, in which Lightfoot accused Cruz of being a “coward Republican” and Cruz criticized the city’s criminal policies as ineffective.

The spat began when Cruz shared a story from Breitbart about the Labor Day weekend gun violence in Chicago, tweeting that disarming “law-abiding citizens” was not the answer, but rather the answer is incarcerating violent criminals before they have the opportunity to commit more crimes.