"If I had a son, he'd look like Trayvon," President Barack Obama said in March 2012 about the shooting death of 17-year-old Trayvon Martin four weeks earlier. In so saying, Obama gave the White House imprimatur to a politically irresistible campaign that both stoked the grievances of his racially sensitive base and energized his party's gun-control advocates. That the shooting took place in Florida, the most highly contested state in that...