In the hours after a gunman shot 12 police officers, killing five in Dallas, Republican presidential candidate Donald Trump and his Democratic rival Hillary Clinton abruptly canceled campaign events. Their Twitter accounts largely went quiet. The shootings marked yet another convulsive event in the 2016 political season, one in which Clinton and Trump have scrambled to find the right response to terror attacks abroad, mass shootings at home, and protests over police killings of African Americans. 'We're seeing wild acts of gun violence and we are polarized in our politics and in the public square,' said historian Douglas Brinkley. 'Every 48 hours there seems to be some horrific event that jars and jerks our consciousness into a new dimension.'