This wasn’t the way Joe Biden wanted the first debate to go

The biggest single question going into the first Democratic debate was whether any candidate would manage to lay a glove on the frontrunner, former Vice President Joe Biden.

Well, Sen. Kamala Harris did that, and then some.

In an exchange that immediately became the standout moment of the two-night event, Harris sharply criticized Biden’s recent musings about his past productive work with segregationist senators. (“At least there was some civility. We got things done,” Biden had said.)

Biden tried to respond by arguing that he fought for civil rights — but Harris fired back, pressing him on the issue of busing in particular, and citing her own personal story. “There was a little girl in California who was part of the second class to integrate her public schools and she was bused to school every day,” Harris said. “And that little girl was me.”

Again, Biden tried to make a distinction. “I did not oppose busing in America,” he said. “What I opposed is busing ordered by the Department of Education.” Harris, however, then responded by saying that the federal government does need to step in to support civil rights if state and local governments can’t or won’t.

At the end of the exchange, Biden ran out of things to stay. “Anyway, my time is up,” he said, trailing off.

This wasn’t the only challenge Biden faced during the debate. Sen. Bernie Sanders pressed him to defend his 2002 vote to authorize the Iraq War (and Biden didn’t respond effectively). Even Sen. Michael Bennet (D-CO) argued that a tax deal the vice president cut with Mitch McConnell wasn’t actually very good.

But the Harris moment is sure to be the one that will replay frequently on cable news in the coming days. And it crystallizes several potential problems for Biden’s campaign.

Biden has a long record to defend in a Democratic Party that has changed: This is a problem that’s similar to the one Hillary Clinton faced in 2016 — Biden is forced to either defend or renounce positions he held decades ago that are no longer viewed as acceptable in the party today. His role in the busing battle is a prime example of that.

It’s unclear whether Biden will retain his strong support among of black voters: Biden has led almost every national and early state primary poll so far this year — and part of the reason for that has been his strong support among black voters, who make up a sizable portion of the Democratic electorate. And one major question for Biden’s campaign is just how rock-solid that support is. His exchange with Harris will be its first serious test.

Biden’s mouth can get him in trouble: The main reason Harris had this plum opportunity was Biden put his foot in his mouth at a fundraiser last week. He’s long had an unguarded way of speaking: going off-script, embarking on lengthy riffs, and sometimes saying things he ends up regretting. Sure, he can argue that his comments are being misrepresented, but Biden’s campaign opponents are certainly not going to go out of their way to read everything he says generously. And if he’s provided fodder like this for Kamala Harris to attack him, it sure seems plausible that he’ll provide fodder for Donald Trump too.

It’s still quite early in the Democratic race, so it’s unclear if this debate night exchange will truly change the race. Perhaps Biden’s support base will stick with him. Perhaps most voters still aren’t paying all that much attention. But there have long been doubts within the party about Biden, so Democrats will surely be eyeing polls in the coming days to see if they show any signs of weakness.