hours gunman shoot 12 police officer kill five dallas republican presidential candidate donald trump democratic rival hillary clinton abruptly cancel campaign events twitter account largely go quiet shoot mark yet another convulsive event 2016 political season one clinton trump scramble find right response terror attack abroad mass shoot home protest police kill african americans much violence capture live smartphones endlessly replay cable television news fuel americans fear personal safety poll show evoke memories 1968 civil right leader martin luther king democratic presidential candidate robert kennedy assassinate turbulent era protest vietnam war racial segregation see wild act gun violence polarize politics public square say historian douglas brinkley every 48 hours seem horrific event jar jerk consciousness new dimension historically uncertain time tend push party power witness republican ronald reagan defeat incumbent democrat jimmy carter 1980 iran hostage crisis obama victory republican john mccain 2008 u.s. economy collapse may case trump yet convince majority americans would steady hand tiller trump initial reaction dallas suggest understand stake issue sober statement unity unlike many public utterances often deride critics hyperbolic inflammatory nation become divide many americans feel like lose hope crime harm many citizens racial tensions get bad good american dream want children say time perhaps ever strong leadership love compassion pull tragedies race relations become prominent issue white house race former house speaker newt gingrich top contender trump vice presidential run mate tell fox news trump help broaden appeal go inner cities many african americans live.in respond spasm gun violence clinton call regulations gun industry align black live matter movement campaign violence african-americans round tv appearances clinton say work whole life try bridge racial divide comment follow police shoot two black men two separate incidents louisiana minnesota past week dallas gunman tell police angry kill call white people like put shoe african american families ... fear every time children go somewhere go talk white people clinton say cnn democratic strategist bud jackson say hit right tone think issue need take advantage say fact would probably hurt perceive take advantage it demand change sweep among voters accord reuters/ipsos poll take last five weeks nearly two-thirds americans feel country wrong track reflect general unease economy terror threats violence challenge show cope issue would bring change would say presidential historian thomas alan schwartz trump say clinton win nov. election would amount third term president barack obama clinton embrace many policies former rival 2008 presidential election campaign week lanhee chen advise 2012 republican nominee mitt romney 2016 presidential candidate marco rubio say civil discourse american politics would help calm people general fear think big issue around civility rhetoric discourse leaders appeal appeal good angels really see absence leadership around violence convulse 2016 election campaign test trump clinton