Democrats want to see a candidate who will take it to President Trump. And they’re worried, still stewing in their 2016 loss, that a female candidate or a candidate of color will face a tougher path to the White House. Ms. Harris is both, and part of what she needed to do on Thursday was prove that she could go on the attack. Going after Mr. Biden was a substitute for going after Mr. Trump.

Whether this will provide the boost Ms. Harris needs remains to be seen. In recent weeks, Senator Elizabeth Warren has stolen much of the early excitement surrounding Ms. Harris’s entry in to the race. Her numbers have remained fairly stagnant, and she has struggled to settle on a message.

And there is some political risk: Busing as a means of integrating schools, which aides to Ms. Harris say she embraces, is fairly unpopular. The criticisms are myriad. Most significantly, it hasn’t been successful at desegregating schools, in part because there was no political coalition for it to work.

Mr. Biden’s aides dismissed the attack as unfair, touting his record on civil rights and saying he was not as unprepared as it might have seemed. “He was not interested in engaging in attacks on other Democrats,” said Kate Bedingfield, his deputy campaign manager. “We fully expected attacks to come.”

What is clear after last night, however, is that Ms. Harris’s attack opened a new stage in the race — one in which candidates beyond Mr. Sanders are going after Mr. Biden’s weaknesses far more directly. Senator Michael Bennet on Thursday dismissed Mr. Biden’s skills as a congressional negotiator, as did Senator Kirsten Gillibrand.

On Twitter, Mr. Trump’s campaign manager gloated about Mr. Biden’s performance. Remember, the president was awfully rattled by polls showing Mr. Biden beating Mr. Trump, though his team has been fairly bad at predicting what, exactly, will motivate Democratic voters.

O.K., and with that, I’m turning in. Like Beto O’Rourke, I’m no punk rock kid anymore.

But, one last note: Turns out I was very, very wrong about the level of interest in this debate. The first night exceeded ratings expectations, and Thursday was expected to be even higher. I loved reading the notes from hundreds of you who wrote in with your thoughts after the last newsletter. Keep them coming!