On Sunday, a man armed with an assault weapon marched into a popular pizza place — Comet Ping Pong — in Washington. He said he had come to “self-investigate” false stories spread by hate sites that the restaurant was the center of a Hillary Clinton ring trafficking in children. He reportedly fired his rifle one or more times and was arrested. Luckily, no one was injured.

After the utterly baseless and scurrilous stories spread over social media, the restaurant’s owner and staff had been inundated with death threats. The lunatic conspiracy was spun off of WikiLeaks’ publication of Mrs. Clinton’s campaign chairman corresponding with the owner about a possible fundraiser. The neighbors who know and love the restaurant have rallied to its support. But under the hashtag #pizzagate, the hate stories keep spreading — and the threats keep building.

Mr. Trump, this ugliness is poisonous and dangerous. But it keeps spreading. On Nov. 2, a few days before the election, Gen. Michael T. Flynn, the man you appointed to as your national security adviser, issued a tweet tying Mrs. Clinton, your opponent, to “sex crimes with minors.” The full text reads as follows:

“U decide — NYPD Blows Whistle on New Hillary Emails: Money Laundering, Sex Crimes w Children etc…MUST READ.” It linked to an article on the website “True Pundit.” That article, which didn’t mention Comet Ping Pong, alleged — fantastically — that the New York Police Department had new evidence linking “Clinton herself and associates” to a series of crimes.

I’m told that Gen. Flynn now says he was duped. But how can anyone with common sense choose to spread a report that Mrs. Clinton, the Democratic nominee for president and former secretary of state, was involved in “sex crimes with minors.” This isn’t partisan animus. It isn’t the excesses of the heat of an election. It is simple lunacy.

After the shooting on Sunday, Politico reported that Gen. Flynn’s son, an adviser to his father, tweeted: “Until #Pizzagate proven to be false, it’ll remain a story. The left seems to forget #PodestaEmails and the many ‘coincidences’ tied to it.” He linked to the account of the special projects director of a group called Citizens4Trump. He chose to fan the flames. (Gen. Flynn’s son, Michael G. Flynn, was fired late Tuesday from the Trump transition team.)

I realize that Gen. Flynn supported you when you were the underdog. Having run as an outsider, I understand the deep sense of loyalty and appreciation that builds for those who were with you from the start, when the odds were long, the establishment scornful and the press dismissive. It is to be expected that you will want those you can trust around you in the White House.

But as you know, this campaign — which you have correctly described as “the nastiest” — has fanned anger, division and fear in the country. It is vital, as you have stated, that the country come together.

For this to happen, you must lead. The president is the great teacher. What you say and how you act will set the tone. So I respectfully suggest you reconsider your appointment of Gen. Flynn. It is inconceivable that the national security adviser to the president would traffic a report that Mrs. Clinton was engaged in sex crimes with minors.

Gen. Flynn is not the only appointment that raises fears. U.S. Sen. Jeff Sessions of Alabama, with a long history of venom towards people of color, is not fit to be attorney general. But at least Sen. Sessions must face Senate confirmation. Gen. Flynn is your decision alone.

When people are frightened, hatred can spiral out of control. I’ve witnessed this time and again during the Civil Rights Movement, when citizens in both the South and the North were inflamed into assaults on nonviolent and peaceful demonstrators. Leaders can fan those flames, and they can help calm those fears.

That is why I make this simple moral appeal that you stand clearly against the hate and seek to calm the fears in what you say, how you act and who you put in your administration. Your appointments reflect what you think is acceptable. You draw the line. I implore you to draw it wisely.

The writer is the founder and president of the national Rainbow PUSH Coalition.