Beverly Young Nelson, left, and attorney Gloria Allred hold Nelson’s high school yearbook, which they say was signed by Senate candidate Roy Moore, at a news conference in New York on Monday. (Richard Drew/AP)

Sean Hannity is a case study in why Roy Moore won’t drop out of the Alabama Senate race. During his Fox News show on Tuesday, he gave Moore a 24-hour ultimatum: Provide a good explanation for the multiple allegations of sexual misconduct against you, or I can’t support you.

Within that window, Moore replied by way of an open letter to Hannity that he published online.

“I am suffering the same treatment other Republicans have had to endure,” it began. He denied the accusations of two accusers, Leigh Corfman (who says she was 14 when Moore tried to touch her) and Beverly Nelson (who says she was 16 when Moore assaulted her behind the restaurant where she worked). Moore  didn’t offer any response to the claims made by several other women that he had pursued them romantically while they were still in high school.

Close-up photograph of the inscription. (Los Angeles Times)

He did, though, repeat a claim made on Wednesday by an attorney representing him about an inscription in a yearbook that Nelson showed the media when she came forward with her allegations. The inscription, shown above, includes the letters “D.A.,” which his attorney says weren’t written by Moore (who at the time of the alleged incident was an assistant district attorney).

Moore wrote:

“Those initials as well as the date under the signature block and the printed name of the restaurant are written in a style inconsistent with the rest of the yearbook inscription. The ‘7’s’ in ‘Christmas 1977’ are in a noticeably different script than the ‘7’s’ in the date ’12-22-77.’ I believe tampering has occurred.”

For Hannity, this note was enough to keep him from speaking out forcefully against Moore. When he first issued his ultimatum to the candidate, Hannity specifically mentioned the yearbook inscription. During his show on Wednesday, he read Moore’s letter on the air, declaring that he would leave Moore’s fate up to the people of Alabama. Moore had sowed a little doubt on one part of one allegation — and that was enough space for Hannity to stand down on his criticism.

This is precisely why Moore’s lawyer held a news conference: to similarly raise a question about one facet of one allegation and,  thereby, give Moore’s supporters reason to question all of the allegations against him.

But is doubt about the yearbook inscription warranted? To answer that question, The Post spoke with Mark Songer, a former FBI agent who now works as a forensic document examiner in the private sector in Denver.

Songer wasn’t able to offer any assessment of the validity of the inscription, which is precisely the point: There is not enough information at hand for an outside observer to make such an evaluation.

“No two writings are ever exactly alike,” Songer said. “Handwriting is a complex motor skill of sensory neurological and physiological impulses. After practice and repetition, writers interject their own individual characteristics into their writings which become a pattern of habitual formations that are repeated from one writing to the next.”

In other words, your signature is never exactly the same each time you write it — but each time you write it there are common characteristics that your signatures share. To evaluate whether a questioned signature is your signature, a document examiner would need a lot of other examples of your signature (Songer said he would need five to 10) to have enough evidence to determine whether the questioned signature was valid. For other handwriting, like the rest of the inscription, he would need much more: other examples of your known writing (that is, things proven to have been written by you) that would allow him to evaluate individual words and sentences.

What’s more, Songer said, those known writings would need to be contemporaneous to 1977, the year that the inscription was purportedly written. “Everybody’s writing eventually changes over time,” he said, meaning that a bunch of handwriting from 2017 wouldn’t necessarily provide the necessary information to evaluate writing from 40 years earlier.

This is, Songer said, is why it’s hard to dismiss the inscription as invalid simply on the basis of the two sets of “77’s.”

“If you’re comparing just the number ‘7’ the question becomes: Out of the, what, 7 billion people in the world can that be replicated? And the answer is: Yes,” Songer said, making the point that it’s hard to identify a writer simply by how a seven is written. “You can only make a ‘7’ so many ways — or a ‘1’ or a ‘2’. If there were some kind of unique feature that was added to that number and it’s repeated, and it’s unique, possibly.”

“Just comparing numbers on its own merits?” he continued. “It’s very dangerous for someone to say, ‘Yeah, just based on that, that they’re different, therefore different writer.’ I would never go to that level.”

Moore’s attorneys have asked to review the yearbook itself. There’s good reason for this, Songer said.

“As a handwriting expert, we always want to look at the original vs. a photocopy because with the originals we can microscopically examine it,” he said. “Look at the line quality and determine what kind of instrument was used: Was it a ballpoint pen? Was it a gel pen? In 1977, did they have this writing instrument available at the time?” The ink itself could be tested to see if it was an ink that was available in 1977.

That said, Songer didn’t know that Moore’s team necessarily needed the yearbook.

“The quality of this copy?” Songer said, “I think they could come to a qualified opinion.”

Asked whether there was evidence of multiple writers, Songer indicated that he didn’t have enough examples of Moore’s writing to say with certainty.

“Looking at the yearbook entry,” he said, “it looks pretty spontaneously prepared” — that is, it doesn’t look like the writer stopped and restarted, as though someone were tentative in writing perhaps because they were trying to imitate another writer. “It looks very fluid. I don’t see any indications of unnatural writing.”

“The writing seems consistent with one writer,” he added, though he pointed out that “Old Hickory House” and the second date appear to be different stylistically — though he’d need to see examples of Moore’s hand-printed writing to be able to determine whether it’s authentic. (Moore’s lawyers didn’t provide any examples of non-script writing that might show an inconsistency.)

Songer, the former FBI agent, was applying a different standard of proof to the question than did Hannity. Songer wanted to know whether he could demonstrate without question that the handwriting might or might not be Moore’s. Hannity, and many other Moore defenders, are content with the existence of doubt.

Without more handwriting available for comparison, Songer couldn’t rule out that the inscription had been manipulated, though he also didn’t see strong evidence that it had been. With that doubt lingering, Moore’s political goals have already been met — as they were with Hannity.