It went by pretty quickly, but we witnessed a watershed political moment during Monday night’s presidential debate.

When Democratic nominee Hillary Clinton called out Republica nominee Donald Trump for perpetuating a “racist birther lie” and for his “long record of engaging in racist behavior,” she made history. Although she stopped just short of calling Trump a racist, it was, as The Washington Post’s David Weigel noted, an unprecedented “anvil-drop” moment in presidential debate history. Clinton underscored the utter impotence of Trump’s push, in recent weeks, to broaden his appeal to African Americans while simultaneously demonstrating a newfound dexterity in talking about matters of race.

As this thread started by black Twitter O.G. @metroadlib suggests, Trump’s attempt to respond was greeted with almost uniform derision.

He demonstrated once and for all Monday night that he’s simply incapable of constructively engaging the vast majority of black voters in tone, language or on issues, while Clinton — who has had her own past problems with the black electorate — clearly has figured it out. For yet another election cycle, Trump has sealed his own fate, and the GOP’s, when it comes to reaching black voters.

The moment was teed up, in part, by the way moderator Lester Holt framed a question about Trump’s birther escapades. So much of the discussion has centered on the veracity, or lack thereof, of the myth that President Obama is not an American citizen, or lately on a fight over who started the myth in the first place. Up to now, there hasn’t been much discussion of why birtherism is politically toxic. Holt fixed that by asking:

“Mr. Trump, for five years you perpetuated a false claim of the nation’s first black president was not a natural-born citizen. You questioned his legitimacy. In the last couple weeks you acknowledge what most Americans have accepted for years, the president was born in the United States. Can you tell us what took you so long?”

Not only did Holt’s question short-circuit Trump’s eventual answer — a rehash of his previously debunked claim that he provided a public service with his birther quest by finishing what Clinton operatives started — but it exposed the fact that Trump has no understanding of the severe affront he’s given to Americans in general, and African Americans in particular, by pushing the falsehood.

And it gave Clinton a chance to respond with “racist birther lie”; with an account of the lawsuit that Trump settled with the Justice Department in the 1970s over charges that his organization discriminated against black renters; and to lock arms with the aggrieved first family, when she stated that Trump offended the president, “a man of great dignity” and cited Michelle Obama’s refrain, from the 2016 Democratic convention, that where Trump had gone low, “we go high.”

Republican presidential nominee Donald Trump says he favors "stop-and-frisk." Here's what that means and why the policy is so controversial. (Victoria Walker/The Washington Post)

Add that to Trump’s non-starter push, in recent weeks, for a revival of stop-and-frisk laws that a New York federal judge ruled unconstitutional, his ugly “What the hell do you have to lose?” appeal to black voters and his pathetic credit-taking, during the debate, that at his club in Palm Beach, Fla., there’s “no discrimination against African Americans, against Muslims, against anybody,” and you can see that the 41 days he has left in this campaign will not be enough time to change the way he appears to view black people or the way they view him. He’s completely dismantled any lessons learned from Republicans’ post-2012 election “autopsy” report and ceded the field to Clinton when it comes to the base she needs to turn out in November in order to win.

Clinton still needs voters of color to turn out on Election Day, but her criticism of Trump will resonate with all but the most cynical African Americans. And now he’s left defending his crazed birther argument — a “logic” that never served any purpose other than attempting to delegitimizing the nation’s first black president. Trump had to know that this question was coming. He admitted as much in his long rambling answer, but he was still woefully unprepared.

Democratic presidential nominee Hillary Clinton says implicit bias in policing can have “fatal consequences.” (The Washington Post)

If Trump can’t win over more black or Latino voters, he needs something on the order of 65 percent of the white vote, a threshold no candidate has reached in decades. And if his recent black “outreach” efforts are really aimed, as many believe, at presenting himself as more palatable to moderate white voters, then he blew that in the debate, as well. By not deflecting Clinton’s charge that his political rise was fueled by a racist myth, he squandered a chance to show any introspection on his role in this rotten episode or that he has any capacity to heal racial divisions. Meanwhile, Democrats have quietly cemented their electoral advantage among people of color, in part by learning from Obama how to speak about race in a way that isn’t condescending or tone deaf.

By exposing Trump in front of a record-setting national audience, Clinton didn’t merely score debate points, she went a long way toward permanently cuffing Trump to his “racist birther lie.”