In 1992, recording artist Lisa Williamson, better known as Sister Souljah, opined, “If black people kill black people every day, why not have a week and kill white people?” She also said, “If there are any good white people, I haven’t met them.”

Sensing a political opportunity, Bill Clinton went before Jesse Jackson’s Rainbow Coalition to rebuke Williamson. “If you took the words ‘white’ and ‘black,’ and you reversed them,” the Arkansas governor said, “you might think David Duke was giving that speech.”

Jackson was furious. But Americans were reassured that Clinton really was a new kind of Democrat, not in thrall to the party’s left wing. He was elected president later that year.

American politics could use another Sister Souljah moment. Two, in fact.

The obvious need is among Republicans. In the last week, Donald Trump has renewed his campaign of insult against John McCain, proving that the lowest place on earth isn’t the Mariana Trench — it’s the president’s soul. Yet with the notable exception of Mitt Romney, no Republican of national stature has done much more than clear his throat by way of objection, as if the president were no worse than a bumptious uncle passing gas at a Thanksgiving table.

It won’t do. What Trump said wasn’t rude. It was grotesque. It didn’t just besmirch the memory of the honored dead. It assaulted the feelings of the grieving living. It didn’t just transgress bounds of ordinary decency. It blew a hole (another one) in our already shattered expectations of presidential behavior. It showed that Trump isn’t merely unsuited to leadership. He’s unfitted for manhood.

This would be the moment for a rising Republican leader — Nikki Haley comes to mind — to say, in clear and categorical terms, that is not us. She could give a speech in Sedona, Ariz., noting how John McCain’s values represented the Republican Party at its best: courage and freedom; faith and forgiveness; individualism and engagement.

Such a Sister Souljah moment won’t derail Trump’s renomination. It may even harm the immediate political viability of whoever does the denouncing. But it will send a signal, essential for the party’s long-term health, that a future Republican president will restore honor and dignity to the White House.

Democrats, too, desperately need such a moment if they are serious about winning the election next year.

John Hickenlooper had his chance earlier this month on “Morning Joe” to describe himself, based on his business record, as a “proud capitalist.” He flubbed it. Elizabeth Warren and Cory Booker had a chance to repudiate Representative Ilhan Omar’s anti-Semitism forcefully and without equivocation, just as Nancy Pelosi did. They flubbed it. Kamala Harris had a chance to break out beyond her deep-blue base by opposing the Green New Deal in the name of fiscal responsibility. She flubbed it. Kirsten Gillibrand could have embraced her centrist record as a Blue Dog Democrat rather than run away from it. She’s flubbed it. Beto O’Rourke, Julian Castro and Pete Buttigieg could have bucked the charge that Democrats are becoming an anti-Israel party by showing up at next week’s Aipac conference. They’ve flubbed it.

There’s still time. Trump’s executive order Thursday on campus free speech could provide another opportunity for a Democrat to break with progressive orthodoxy.

Step 1: Criticize the order as an example of federal government overreach to which conservatives usually object. Step 2: Acknowledge that there is a serious problem on the left when it comes to speech, and that protecting the speech rights of all Americans is an irreducible and non-negotiable principle of liberalism. Step 3: Pledge that as president you will restore America’s moral leadership when it comes to defending press freedom, ideological diversity and individual conscience.

How about it, Joe Biden?

The chances of this happening seem slim. And the blowback a candidate would face from the progressive social-media furies would be great.

But the point of a Sister Souljah moment isn’t to ingratiate a candidate to the party base. It’s to demonstrate independence and nerve. And furnish evidence that the candidate is of the center, not the fringe. And that he or she can appeal beyond the Democratic base to Americans who think capitalism is mainly good, anti-Semitism and anti-Zionism are profoundly bad, and the intellectual commissars of the woke left are intolerant bores.

Donald Trump plans to win re-election on the strength of a fanatically devoted base, a prosperous economy, a Mueller report that finds no smoking gun and a Democratic Party that increasingly seems to find nothing wrong with the word “socialism.” It’s not a bad theory of the race, especially if there isn’t a Democrat willing to stand up and prove him wrong.

As for the rest of the Republican Party, they believe they can squeak to re-election with a president who extols scoundrels, defames heroes, lies without compunction, holds to no higher principle than his personal gratification and thinks he can always get away with it, because he always does.

Is there no Republican willing to stand up to all this, lest Trump be proven right?