Jack Smith

That’s why Mr. Smith brought the charges in Florida. He thinks the case is strong enough that a jury will convict Mr. Trump even in a jurisdiction of diverse party affiliations. His confidence may also explain why he alleged that Mr. Trump willfully rather than negligently mishandled classified material. He might also have wanted to distinguish Mr. Trump’s case from those of Hillary Clinton, Joe Biden and Mike Pence, none of which allegedly involved willfulness.

What should have begun as a routine civil investigation under the Presidential Records Act has ended up with a multicount criminal indictment, the first federal prosecution ever of a former president or a leading candidate for the presidency. This is partially because prosecutors targeted Mr. Trump and partially because of the unwise way he responded.

Mr. Bragg campaigned for his office on a promise to hold Mr. Trump accountable and delivered when he persuaded a grand jury to hand up a weak indictment. Mr. Smith was appointed specifically to investigate Mr. Trump, and he did his job well. The problem inheres in the office of special counsel, which by its nature selects its target and looks for evidence against him.

Mr. Smith had a lot of help from Mr. Trump. Had the former president cooperated with investigators and immediately returned all the classified material in his possession, as Messrs. Biden and Pence did, charges would have been unlikely. But Mr. Trump did what he always does. He attacked Mr. Smith and resisted his efforts. That provoked investigators to double down, which in turn led Mr. Trump to engage in the allegedly obstructive conduct that forms the basis for several counts in the indictment.

Mr. Smith subpoenaed Mr. Trump’s lawyers and persuaded a judge that Mr. Trump had vitiated the attorney-client privilege by instructing them that it would be “better if there are no documents.” The defense team will claim that Mr. Trump was entitled to maintain possession of classified material under the Presidential Records Act of 1978, which establishes detailed procedures for handling the records of former presidents and a civil process for resolving disputes about them. It doesn’t carry criminal penalties for noncompliance. Remarkably, the indictment never mentions the Presidential Records Act, despite its apparent relevance to any possible prosecution under the Espionage Act of 1917.

The indictment quotes tape-recorded conversations that form the basis for several charges under the Espionage Act. The critical recording is of a conversation between Mr. Trump, a writer, a publisher and two Trump staffers, who were discussing a claim that a senior military official had persuaded Mr. Trump not to order an attack on “country A,” which in context is surely Iran. Mr. Trump points to some papers he found and tells his guests they prove that military officials supported an attack. “This totally wins my case,” he says. “This is secret information. Look, look at this.” Mr. Trump then says: “See, as president I could have declassified it. . . . Now I can’t, you know, but this is still a secret.”

It is possible that Mr. Trump merely waved the papers in front of his guests and never gave them an opportunity to read them, which is apparently not in evidence because the prosecutors don’t have the document. But even those hypothetical facts would be enough to

The reason this recording is so powerful is that it is self-proving. It doesn’t rely on testimony by flipped witnesses or antagonists of Mr. Trump. It is the kind of evidence every defense lawyer dreads and every prosecutor dreams about. This is particularly important because an appellate court could find legal error in the ruling that Mr. Trump had vitiated attorney-client confidentiality and reverse convictions based on his lawyers’ compelled testimony. A conviction that rests on a consensually recorded conversation would be harder to challenge.

Mr. Smith has made a stronger case against Mr. Trump than many observers, including me, expected. The question remains: Is it strong enough to justify an indictment of the leading candidate to challenge the president in next year’s election? Even with the recorded statements, this case isn’t nearly as strong as the one that led to President Richard Nixon’s resignation in 1974. Nixon was almost certainly guilty of destroying evidence, bribing witnesses and other acts of obstruction. Many of the charges in this case are matters of degree. Nor have prosecutors any evidence that Mr. Trump’s actions damaged national security more than those of Mr. Biden, Mr. Pence and Mrs. Clinton did.

When an incumbent administration prosecutes the leading candidate against the president, it should have a case that is so compelling that it attracts the kind of bipartisan support that forced Nixon to resign. No such support is currently apparent, since many Republicans continue to be troubled by the targeting of Mr. Trump. Mr. Smith will have to convince not only a Miami jury but the American public, on both sides of the partisan divide.

Mr. Dershowitz is a professor emeritus at Harvard Law School and author of “Get Trump: The Threat to Civil Liberties, Due Process and Our Constitutional Rule of Law.” He represented Mr. Trump during his 2020 impeachment trial.