In the history of presidential scandals, it is often the hidden things that end up proving decisive. Think, for example, of Monica Lewinsky’s stained blue dress or of Richard M. Nixon’s secret Oval Office tapes.

But in President Donald Trump’s recent scandal involving Stephanie Clifford, the pornographic film star known as Stormy Daniels, something that was never there to begin with could play an unexpected role.

The missing item is the signature Trump failed to place on Clifford’s nondisclosure deal two years ago. And if her lawyer has his way, there is a chance that the inch-long blank space could force Trump to testify about what he knew of the arrangement.

As the world found out in more detail last week, the man who structured the hush-money payment, Trump’s former lawyer and fixer, Michael D. Cohen, pleaded guilty to several charges in federal court.

Under the arrangement, Clifford was paid $130,000 just before the 2016 elections to not speak publicly about an affair that she said she had with Trump. In admitting to the scheme, Cohen not only implicated himself, but also possibly his former boss, in a violation of federal campaign-finance law.

It remains unclear, however, whether Trump, as president, can be held accountable for that offense.

That is where the missing signature comes in.

This year, Clifford’s lawyer, Michael Avenatti, filed a lawsuit against Trump and Cohen, claiming that the nondisclosure contract was “null and void” because Trump left empty the line where he was meant to write his name.

The lawsuit was stayed this spring after federal agents raided Cohen’s office and apartment, prompting him to invoke his Fifth Amendment right against self-incrimination to avoid having to testify in the suit. A federal judge agreed that if Cohen became a witness in the lawsuit, he could expose himself to legal risks in the FBI’s investigation.

The whole thing might have been dismissed as just another publicity stunt from Avenatti, who has a penchant for making provocative statements about Trump and who has even mused about running for president in 2020.

But now that Cohen has pleaded guilty, he may no longer be able — or choose — to avail himself of the Fifth Amendment’s safeguards. Because of that development, Avenatti vowed last week to ask the judge to lift the stay and to let him question Trump about Clifford by taking his deposition.

“The developments of today will permit us to have the stay lifted in the civil case & should also permit us to proceed with an expedited deposition of Trump under oath about what he knew, when he knew it, and what he did about it,” Avenatti wrote on Twitter on the day Cohen pleaded guilty. “We will disclose it all to the public.”

The legal maneuver arguably remains something of a long shot for Avenatti. Still, if he is eventually permitted to depose Trump, he could direct his inquiries toward several unanswered questions that have lingered at the heart of the hush-money arrangement:

Was Cohen indeed reimbursed, as prosecutors claim, by the Trump Organization? Which executives there were involved in the transaction? Was the payment made, as Cohen noted in his guilty plea, to influence the election?

The nondisclosure agreement silencing Clifford was dated Oct. 28, 2016, and was meant to be signed by four people: her first lawyer, Keith Davidson; Cohen; Clifford herself (who was identified as “Peggy Peterson”); and Trump (who was identified as “David Dennison.”)

While it may be curious that Trump never signed the deal, the omission squares with his latest explanation for his role in the imbroglio.

After denying for months that he slept with — or paid — Clifford, the president said last week that even though he was the source of the money, Cohen struck the deal without his knowledge and that he learned about it only after it was inked.

Charles J. Harder, a lawyer representing Trump in the lawsuit, did not return phone calls Monday seeking comment.

Legal experts were divided about whether the missing signature was sufficient legal ground to take Trump’s deposition.

“This is exactly the kind thing that Avenatti can take a deposition about,” said Debra Katz, an employment and whistleblower lawyer based in Washington who has negotiated several nondisclosure deals. “Because there’s no criminal case pending now and Cohen is out of jeopardy, it would be appropriate for the court to lift the stay and let the case go forward.”

But Douglas Wigdor, a New York lawyer who has represented several people — some with nondisclosure deals — who have sued Fox News for sexual harassment, said it was unlikely that the president could be deposed. First, Wigdor said, Cohen could ask for the stay to remain in place because his guilty plea does not immunize him from other potential charges, and he could thus still invoke his Fifth Amendment rights. It also often happens, Wigdor said, that executives of companies are sued, and the companies themselves negotiate the deals and pay the settlements without the executives ever being involved.

As long as Clifford was paid, Wigdor added, “Trump could have known or not known — signed or not signed — it doesn’t matter.”

Wigdor pointed out that a separate lawsuit that Avenatti filed in June, accusing Cohen and Davidson of colluding to quash Clifford’s tale of an affair with Trump, stood a better chance of resulting in depositions.

If it could be proved, Wigdor said, that the two lawyers did collude with one another — especially for Trump’s benefit — then Avenatti would have “a stronger argument” for asking Trump what he knew about the deal. Dolores Troiani, a lawyer who helped Andrea Constand overcome a nondisclosure agreement to testify against Bill Cosby, leading to his sexual-assault conviction this year, suggested that the smartest move for Trump and his team would be to nullify Clifford’s contract on their own.

The president could be deposed, Troiani said, only if he continues seeking to enforce the deal.

“If there is no contract, then no one is breach of it — and it’s done,” she said. “My advice? Just let it go.”