When you’ve been in politics and in Washington as long as he has — 36 years in the Senate, plus eight as vice president — there are votes and quotes from eras much different from the current one, controversial positions galore and mistakes aplenty. All of these were ammunition used against him on Thursday night, most electrically when Harris pressed him to defend his opposition to busing to integrate schools.

Harris made it personal, telling him that she got the education she did because of busing. Biden said that he hadn’t been opposed to busing so much as in favor of local decision-making, and he thus left himself open to her righteous response: Did he not think that the federal government should swoop in to remedy obvious racial injustice?

“That’s why we have the Voting Rights Act and the Civil Rights Act,” she said. “Because there are moments in history where states fail to preserve the civil rights of all people.”

What happened across the two nights of Democratic debates was fascinating. It ratcheted up the suspense of the nascent Democratic contest; it underscored the difficulty of figuring out the toughest adversary for Donald Trump.

The supposedly safest or most tested candidates (Biden, Sanders) proved to be the least exciting ones. The moderates (Amy Klobuchar, Michael Bennet, John Hickenlooper) couldn’t quite break through. And none of the top five performers — Harris, Buttigieg, Elizabeth Warren, Julián Castro and Cory Booker — fit the demographic profiles of presidents past. Two of them are women, three are people of color and the one who is neither of those things is gay.

Would that make them risky nominees or bold ones? The stakes of answering that unanswerable question correctly are enormous.

All of them except Warren are under 60, and the generational divide between them and Biden, 76, and Sanders, 77, was stark during Thursday night’s event, partly because Buttigieg, 37, and Eric Swalwell, 38, made sure to highlight it.