A dozen years after Barack Obama’s presidential candidacy led Democrats to dream of a post-racial America, toxic issues involving segregationists and a shooting by police are casting a shadow over the 2020 campaign.

Going into the first Democratic primary debates in Miami, issues of race loom over several first-tier candidates, each of whom is trying to court black voters ahead of the early nominating contests. The issues span the generations of Democratic candidates, from Pete Buttigieg, 37, mayor of South Bend, Indiana, to Joe Biden, 76, the former vice president.