Another set of potential issues raised by Mr. Cohen’s admission centers on legal ethics. As a New York lawyer, he is subject to rules of professional conduct in that state and could face disciplinary action — ranging from private admonishment to losing his license to practice law — if he violated them, although the still-uncertain details make it impossible to precisely identify which standards apply.

“We don’t have the facts,” said Philip Schrag, a Georgetown University law professor and specialist in legal ethics issues. “There is an awful lot of speculation here about the facts, which we need before we can go into the legal analysis.”

Depending on what the circumstances turn out to be, he and other ethics specialists pointed to Rule 1.8, which governs financial arrangements between a lawyer and a client. One provision generally prohibits lawyers from lending money to clients during contemplated or pending litigation, although Ms. Clifford appears to have been threatening to talk — not to sue.

Another provision requires a written and signed agreement if there is a loan between a lawyer and a client. If it turns out that the arrangement was envisioned as a loan that Mr. Trump would eventually repay, Mr. Cohen was obliged to create such a document.