How did the most sensitive part of Donald Trump’s anatomy become a talking point in the presidential election?

Blame Graydon Carter.

Nearly thirty years ago, when he was the co-editor of Spy magazine, Carter called Trump a “short-fingered vulgarian.” According to Carter, Trump has never forgotten the insult.

Carter, now the editor of Vanity Fair, resurrected it late last year as Trump surged ahead in the polls. Trump’s allegedly small hands have become a favorite target of Trump’s critics — allowing for an easy segue to other parts of his anatomy.

Marco Rubio brought it up on the campaign trail last weekend. So that’s why Trump seemingly invoked his hand size — and penis size — in a shocking moment during Thursday night’s GOP debate.

“He hit my hands. Nobody has ever hit my hands. I have never heard of this,” Trump said. He held up his palms to the cameras and said look: “Are they small hands?”

The audience laughed approvingly.

Trump then pivoted back to Rubio and delivered what was, according to Facebook, the “top social moment” of the debate: “And he referred to my hands, if they are small, something else must be small. I guarantee you there is no problem. I guarantee.”

The Fox moderators didn’t acknowledge what was almost universally agreed to be a self-assessment of his penis.

But it was an OMG moment among professional commentators and ordinary viewers.

“American politics has crossed the Rubicon of penis size,” The New Republic editor Gabriel Snyder tweeted

“Hope children aren’t watching this,” said historian Michael Beschloss.

Trump’s remark was seen by some as the ultimate assertion of strength, of virility, in a campaign premised on winning.

To others, it was a sign of insecurity.

Trump said on stage that he had “never heard of” the small hands insult, but according to Carter’s editor letter in last November’s Vanity Fair, Trump has never forgotten it.

The “short-fingered vulgarian” line first appeared in print in 1988, when Carter and Kurt Andersen were running “Spy,” a satirical magazine.

“To this day, I receive the occasional envelope from Trump,” Carter wrote last November. “There is always a photo of him — generally a tear sheet from a magazine. On all of them he has circled his hand in gold Sharpie in a valiant effort to highlight the length of his fingers. I almost feel sorry for the poor fellow because, to me, the fingers still look abnormally stubby.”

Carter said he received a “See, not so short!” message from Trump as recently as early 2015.

In January, the bodily critique gained new life among a group of conservative Trump opponents. Some of them shared pictures of Trump’s hands and compared them to baby hands.

Rubio invoked it last Sunday during a campaign stop in Virginia.

“He’s like 6-foot-2” but “his hands are the size of someone who’s 5-2.”

Rubio mocked Trump and set up a penis joke, saying, “You know what they say about men with small hands?” He paused, let the crowd cheer, and said, “You can’t trust ’em!”

That same day, HBO host John Oliver brought up the “short-fingered vulgarian” line and Carter’s assertion that Trump has never forgotten about it.

“Look, his fingers seem fine,” Oliver said, “But the very fact he’s so sensitive about them is absolutely hilarious.”

Later in the segment, Oliver referred to Trump’s “cocktail sausage fingers” and “tiny, tiny fingers.”

All of this inspired The Washington Post to conduct a tongue-in-cheek investigation. For the record, its conclusion was that Trump “is not short-fingered.”