The investigation of Michael Cohen, a lawyer and sometime fixer for Donald Trump, marks the start of a new phase in the unraveling of Trump’s presidency. Until now, the only Justice Department investigation of Trump has been conducted by special counsel Robert Mueller. Mueller’s appointment letter authorizes him to investigate Russian interference in the 2016 election and crimes arising from it. Any wrongdoing that is not connected to this interference arguably falls outside Mueller’s scope. In contrast, the Cohen investigation is taking place not under Mueller, but under the authority of the US attorney for the Southern District of New York. The Southern District is not restricted by any appointment letter, and its career attorneys have a mandate to uncover any criminal activity within their jurisdiction, whenever it may have occurred and for whatever purpose.

In the Cohen investigation, the FBI and federal prosecutors are seeking to break the confidentiality of Cohen and Trump’s attorney–client relationship. To do this, they must either show that Cohen was not genuinely acting as Trump’s attorney or that the two men’s communications were made with the intention of committing or covering up a crime or fraud. The fact that the Southern District prosecutors have already convinced a judge to issue a search warrant on one or both of these bases means they are likely to get access to much material that would otherwise have been unavailable because of attorney–client privilege.

Longtime Trump observers immediately began speculating that the prosecutors will find information that could implicate Trump himself in criminal conduct—or that Cohen could testify against Trump in exchange for a reduced sentence.1 Potential crimes start with the possibility that Cohen violated federal election law, if he paid the porn actress Stormy Daniels $130,000 to remain silent about her affair with Trump without receiving repayment from Trump or the Trump campaign. The payment could be deemed an unreported donation to the campaign, which is a federal crime. Other possible crimes include structuring payments to avoid bank-reporting requirements—a commonly charged felony—or money-laundering related to the purchase and sale of Trump properties.

The legal route to holding Trump criminally liable for Cohen’s actions lies in the law of conspiracy. Since Cohen acted as a fixer for Trump, investigators could discover that on some occasions he acted criminally on Trump’s behalf. If so, a prosecutor could charge that Cohen and Trump were engaged in a criminal conspiracy, defined as an agreement to commit crimes in concert. Once two or more people have agreed to the general plan of action, any crime committed to advance the conspiracy counts as a crime for which both are criminally responsible—even if one acts without the other’s knowledge. For example, if Trump and Cohen criminally agreed to intimidate Stormy Daniels, and Cohen arranged for her to be threatened, then Trump could be charged with that threat, even if he did not know anything about…