President Trump needs a new defense. He started out with the claim that he didn't know anything about payments that his former lawyer Michael Cohen arranged or made to the former Playboy model Karen McDougal and the pornographic film actress Stormy Daniels. Mr. Cohen's tape destroyed that assertion, at least as it applied to Ms. McDougal.

His defense then evolved to denying any personal responsibility for those actions. Predictably, Mr. Cohen's plea agreement on Tuesday destroyed that defense, too, after he told a federal judge under oath that the president had directed him to arrange payments to the two women, who claim to have had affairs with Mr. Trump, "for the principal purpose of influencing the election."

Now President Trump has moved on to a new defense, claiming that what he did wasn't a crime and so it can't be prosecuted. That argument has no basis, either, and is inconsistent with centuries of Anglo-American law.

Here's the new Trump argument, stripped down to its essence: It was clear that he would reimburse Mr. Cohen for those payments to the women, and he's allowed under Supreme Court precedent to give his campaign as much of his own money as he wants to.The problem is that legally, his argument doesn't get him where he wants to go. Even though Mr. Trump can give his campaign as much of his own money as he wants to, he can't ask other people to front the money for him and promise to pay them back later without reporting the arrangement in a timely fashion to the Federal Election Commission. But he didn't report it, subverting the whole point of the nation's post-Watergate campaign finance laws, which is to disclose campaign giving and spending to the American people before an election — not 20 months later.

But, the Trump defenders say, reporting violations happen all the time, and that is certainly true. But there are two facets that make the Trump reporting violations criminally significant, as opposed to a misdemeanor oversight or bureaucratic snafu: It appears to have been an intentional end-run around the campaign finance laws and to involve a conspiracy. Each of these points explains why the new Trump argument will fail.

Criminal law focuses on mens rea, or criminal intent. This means the very same act can be criminal if done with one state of mind and innocent if done with another. It is a mistake to think about Mr. Cohen's allegations as some sort of routine paperwork error. Structuring a transaction to intentionally avoid reporting it as required by the law is a very serious offense, not a technical one that can be forgiven. That is particularly true of the secret payments to the two women, which, had they been disclosed before the election, as they should have been, might have altered the outcome.

The second facet is even more problematic for the president. Prosecutors use the conspiracy doctrine to punish two or more people who merely agree to commit a criminal act. They don't even have to actually perform the act; they just need to have agreed to do so. The idea behind conspiracy liability is that when two people agree to commit a crime, it's much worse for society than when a lone actor does. A Yale Law Journal article I wrote on this subject was inspired by a riddle: Why is it that if you sell a joint, you get a six-month sentence, and if your friend sells a joint, he gets a six-month sentence, but if you both agree to sell a single joint, you get a five-year minimum sentence? The outcomes seem really odd because it looks as if the same crime is getting different punishments.

The answer is that it isn't the same crime, and hasn't been thought of that way in the Anglo-American legal tradition for over 500 years. Rather, conspiracy has always been a separate offense, punished independently without calibration to the underlying crime. So conspiracy to sell a joint can be punished the same way as conspiracy to sell a kilo of marijuana.

Why would the law be written that way? The answer has to do with the harm to society when individuals agree with one another to commit criminal acts. These acts are seen as possessing a higher level of moral culpability and are also more dangerous. Two people can often do more harm than one. And those criminal economies of scale are sometimes supplemented by psychological dangers. People tend to take more risks in groups than alone. For these reasons, the law has always treated conspiracy harshly. Indeed, for much of American history, conspiring to commit an immoral but not illegal act was itself punishable as conspiracy.

That is why the latest Trump defense has no viability. His defenders say there is no precedent for a campaign finance reporting violation being punished as a serious felony. Even if that claim were true, and it isn't, they are looking at the wrong precedents. After all, Mr. Cohen has pleaded guilty to making or facilitating illegal campaign contributions and has said the president directed him to do so, suggesting that Mr. Trump was a co-conspirator in those crimes. And even assuming we were dealing with just a reporting violation, the right precedents are the thousands of cases in America where even low-level crimes have been severely punished because they involve intentional conspiracies.

Incidentally, it's no surprise that Mr. Trump himself came out in an interview aired Thursday against the practice of "flipping," where prosecutors give a guilty person a deal in exchange for information against another person. Flipping and conspiracy charges go hand-in-hand; the latter is what encourages the former.

We are approaching a reckoning, where criminal and perhaps impeachment processes will begin asking hard questions. It would be a huge mistake for the president to rely on assurances from his legal team that what he did was ordinary and not prosecutable. Rather, if the Cohen allegations are true, what President Trump did was knowingly conspire to violate federal campaign law and to hide it from the American people right before the election, and that very severe crime is one that must be punished.

Neal Katyal, an acting solicitor general in the Obama administration, is a law professor at Georgetown and a partner at Hogan Lovells. This was published on August 23, 2018, New York Times.

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August 26, 2018