REGARDING THE tawdry story of President Trump’s involvement with Stephanie Clifford, a.k.a. Stormy Daniels, it’s important to distinguish the aspects that present legitimate matters of public interest from those that do not.

Dishonorable as it might have been, an extramarital affair 12 years ago between then-private citizen Donald Trump and Ms. Clifford belongs in the latter category. A $130,000 hush-money payment from Mr. Trump’s lawyer to Ms. Clifford in the waning days of the 2016 campaign, however, raises some pertinent issues. First, there’s the possibility, admittedly legally debatable, that the payment was intended to influence the election and thus constituted an undisclosed — i.e., prohibited — campaign donation. More significant, though, is the question of where, exactly, that $130,000 came from. The only thing worse than a president subject to blackmail from a former partner in an illicit affair would be a president secretly beholden to a person who paid a huge amount of money to save him from being blackmailed about that affair.

In that regard, Mr. Trump only exacerbated the problem by flatly claiming Thursday that he did not know in advance about the hush-money agreement and that he did not know the source of the $130,000. After these statements, only two possibilities remain. First, Mr. Trump could be lying: He did, in fact, know about his lawyer Michael Cohen’s efforts to buy Ms. Clifford’s silence, and he did eventually reimburse Mr. Cohen for the $130,000 the lawyer says he facilitated out of his own funds, or knows who did. Second, Mr. Trump is telling the truth, and someone else, whose identity remains unknown, both to him and to everyone but Mr. Cohen, put up the cash, thus acquiring secret but substantial leverage over the president of the United States.

Neither of these alternatives remotely corresponds to the ethical standards we expect a chief executive to meet. Ironically, the first — that Mr. Trump is lying — at least has the advantage of being consistent with most of the known facts, which strongly imply that he did have a dalliance with Ms. Clifford and that he has been desperately trying to cover it up, just as many other powerful men before him have tried to cover up their affairs. Come to think of it, if Mr. Trump has nothing to hide, why is his lawyer fighting to keep the legal dispute with Ms. Clifford behind the closed doors of an arbitration proceeding rather than let it play out publicly in court?

The American people do not have a right to know all the details of what went on between Mr. Trump and Ms. Clifford in their personal lives many years ago. They do have a right to know, however, whether their president is lying to them now, or if he has received what amounts to a large financial subsidy from a secret personal benefactor. Unless and until Mr. Trump directs his lawyer to identify the source of the $130,000, both of these sorry scenarios will remain within the realm of plausibility.