The common explanations -- Clyburn's endorsement was the game-changer ("you brought me back," Biden told him), the center-left coalescing behind his candidacy and African-Americans recognizing his close ties to Barack Obama -- all tell a big part of the story.

Equally important, yet sometimes overlooked in the punditry's relentless nose-against-the-window focus on the news of the hour, is that in an era of nearly relentless nastiness, Biden exudes decency. And his biography makes him an almost uniquely sympathetic figure on the national stage. That enduring reality -- the fact that so many voters who almost reflexively dislike all politicians have a reserve of affinity for Joe -- must be central to almost any understanding of his remarkable Lazarus-like resurrection.

It is hard to think of another major political figure who lost both his first wife Neilia and two children 43 years apart: Neilia and baby daughter Naomi to a traffic accident in 1972, his adult son Beau to glioblastoma, a brain cancer, in 2015. His pain has enabled him to connect with people at a gut level, giving him the type of empathy that political leaders rarely manage to tap with so much poignancy.

And Biden's life's work -- his calling to politics and public service -- is something that he seems to need and feed off. Politics has given him an outlet and perhaps a hunger for personal connection where he has been able to channel his grief.

George W. Bush's campaign manager Ken Mehlman once observed that presidential campaigns "are about attributes, not issues."

Biden's political resurgence has to do as much with his solace, compassion and pain as with his endorsements, experience and good political fortune. His traits also match up well with Trump, who, as president, has shown virtually no humanity. And they match up well with Bernie Sanders, whose bombastic thesis that Biden personifies a rigged system is belied by Biden's life of service, tragedy and compassion.