At the second Democratic debate, Kamala Harris made her move and called Joe Biden a racist while saying Joe Biden is not a racist. He disagreed with her forcefully — but fearful that he would seem like the logorrheic lunatic known to everyone in Washington, he noted that his time was up and stopped talking.

Stopped talking! Joe Biden! He must be old! He must be out of step! He must be defensive!

And that was it. Within 30 seconds, left-liberal Twitter declared Biden a political corpse and began drawing up blueprints for Harris’ presidential library.

Without question, Harris is the star of the week. Before this, she was the most mystifying candidate in the race; it was hard to understand just why this smart, savvy, wildly successful politician from the nation’s largest state seemed to be mired at 7%.

She made the case for her candidacy with the brio and aggression that was missing from her months on the stump.

Still: Whoa there, Nellie.

There’s a madness that attends the insta-reactions on social media, as well as a collective groupthink that feeds on itself and in minutes creates conventional wisdoms that have no basis in any reality other than the collective madness of the crowd.

Last week people told us that Biden had been mortally injured by his remarks about segregationist senators — and then the polling that followed suggested he hadn’t been injured at all.

In fact, Biden made no gaffes Thursday night. He was attacked and he dealt with the attack. The attacker may have had the better of the exchange, but that doesn’t mean he was erased from the future of the Democratic primary.

The likeliest outcome of the week’s proceedings is that Biden will remain comfortably on top, while Harris will rise into double digits — and not at his expense.

Let’s be serious. Do most Democratic voters really believe Biden is a segregationist-loving secret racist? That was the substance of Harris’ attack.

Maybe millennial Democrats who believe every white person over 30 is a racist are happy to accept this implicit charge, but will the vast majority of Democratic voters accept such a portrait of Barack Obama’s vice president?

If they don’t, then the question goes to style. Biden’s assets are the notions that he is the most electable person in the field and that he is the sanest person in the field.

Did Harris make the case not only for herself as a performer but also as someone who could outperform Biden against Donald Trump?

Did Pete Buttigieg, who was the night’s other standout performer, make the case that he was at least as sane as Biden?

Maybe. Maybe even likely so. But one of them has to combine the two. Biden’s central appeal is intact: He’s the guy who will end the crazy, not only the Trump crazy but implicitly the socialist crazy that was bellowed every five seconds by Bernie Sanders.

You’ve heard of mansplaining? Bernie was yellsplaining. If I screamed as much as he screamed in that debate, I would be in the emergency room at the Ear Nose and Throat Hospital and would be put on total vocal rest for six months. You have to give the old guy credit. He’s got world-class pipes. He’s like the Pavarotti of Commies.

Quick notes: New York’s own Kirsten Gillibrand claimed at one point that she supported single-payer health care when she was a member of Congress in a conservative district. That was just a bald-faced lie.

Marianne Williamson was the best psycho candidate since the Rent Is Too Damn High guy.

There’s a guy named Eric Swalwell who was on the stage. He was so obnoxious I’m wondering whether someone gave him a wedgie in the men’s room after the debate was over.

Finally, there was so much policy insanity on these stages the past two nights — including the idea expressed by several people that we shouldn’t actually deport illegal immigrants who commit violent crimes — that this party is going to hand a second term to Donald Trump if it doesn’t wise up and quick.