His rivals, beginning with Cory Booker and Bill de Blasio, have already started to pounce.

By any standard, Joe Biden is the Democratic presidential front-runner. The poll averages at RealClearPolitics, for example, show Biden with a commanding 32–15 lead over Bernie Sanders in national polls and leading Sanders by 27 percentage points in South Carolina, 13 in New Hampshire, 13 in Nevada, and six in Iowa. FiveThirtyEight shows Biden leading Kamala Harris in the race for endorsements. The bedrock of Biden’s support is African-American voters, among whom he led Sanders 50–10 in mid June in a national Economist/YouGov poll, and 50–13 in an early-June battleground-state CBS/YouGov poll, and led Elizabeth Warren 52–14 in a mid June Charleston Post and Courier poll of South Carolina. Most national polls show a similar picture.

There are obvious reasons that Democrats in general and black voters in particular are, for now, with the 76-year-old Biden. He carries a lot of good will from being Barack Obama’s devoted vice president; relations between the two were visibly warm. Biden’s name recognition and establishment support go a long way, too. For Democratic partisans focused mainly on beating Donald Trump, much can be forgiven of a guy who was on two winning national tickets and is seen as the “electable” candidate who can reach moderates and blue-collar white Midwesterners.

But in a political culture increasingly focused on past sins against racial and gender equality, Biden has long been sitting on a time bomb: his many enduring friendships and alliances with segregationist Dixiecrats. And by “segregationist,” I don’t mean “not totally on board with the progressive agenda circa 2019”; I mean the real-deal: signers of the pro–Jim Crow Southern Manifesto, “massive-resistance” bitter-enders, raisers of the Confederate flag on public property, you name it. The Senate Democratic caucus Joe Biden joined in 1973 was headed by a former Klansman, with notorious segregationists running virtually all the major committees. These men became Biden’s mentors and friends, and he had nothing but glowing words for them his whole career.