In the history of presidential scandals, it is often 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 Trump’s recent scandal involving Stephanie Clifford, the pornographic film star known as Stormy Daniels, something that was never there to begin with could soon play an unexpected role.

The missing item is the signature that Mr. Trump failed to place on Ms. Clifford’s non-disclosure deal. If her lawyer has his way, there is a chance that inch-long blank space could force Mr. Trump to testify about what he knew of the arrangement.

Last week, the man who structured the payment, Mr. Trump’s former lawyer and fixer, Michael D. Cohen, pleaded guilty to several charges in federal court. Under the arrangement Mr. Cohen helped make, Ms. Clifford was paid $130,000 just before the 2016 elections to not speak publicly about an affair she said she had with Mr. Trump.