It’s long been clear that Presidents can be impeached for “high crimes and misdemeanors” that are not actual violations of federal criminal law. In an oft-cited passage from Federalist No. 65, Alexander Hamilton wrote that impeachable offenses “are of a nature which may with peculiar propriety be denominated POLITICAL, as they relate chiefly to injuries done immediately to the society itself.” They involve “the misconduct of public men, or, in other words, from the abuse or violation of some public trust.” This, at the moment, is the core of the case against Donald Trump for his interactions with the President of Ukraine—that he abused his power by using taxpayer dollars as a tool to extract information potentially damaging to a political rival.

But, if Trump’s behavior was an abuse of power, was it also a crime? The leading candidate for a relevant criminal statute is a familiar one in the federal courts, called the Hobbs Act. The law, named for the Alabama congressman who sponsored it, was enacted in 1946. It prohibits what’s known as “extortion under color of official right.” But what does that mean in plain English?

Samuel W. Buell, a professor at Duke Law School who is a former federal prosecutor and the author of “Capital Offenses: Business Crime and Punishment in America’s Corporate Age,” said, “The traditional way the Hobbs Act is used is when public officials solicit bribes. The idea is that there is an inherent power relationship between a public official and people who need things from that official. If the public official demands money, that’s seen as extortion, and thus a violation of the Hobbs Act.”

So what does that have to do with Trump and Ukraine? “The idea behind the case would be Trump conditioned the release of military aid to Ukraine on the President of Ukraine coming across with the dirt on the Biden family,” Buell said, adding, “He’s misusing official power to obtain things of value to him. That’s the heart of what the Hobbs Act is supposed to prohibit.” Buell draws an analogy to the Hobbs Act prosecution of Rod Blagojevich, the former governor of Illinois. “Lobbyists for a children’s hospital wanted Blagojevich to increase Medicaid reimbursement rates, which meant eight million dollars in revenue to the hospital,” Buell said. “But he put out the word through intermediaries that he would only do it if he got fifty thousand dollars in campaign contributions. That quid quo pro was a violation of the Hobbs Act. With Trump, the quid pro quo is taxpayer money in return for political dirt, but the idea is the same.”

There are problems with this theory, starting with the President’s constitutional prerogatives to conduct foreign policy under Article II. Trump, or his lawyers, could argue that such a case would criminalize the give-and-take of negotiation with foreign governments. International negotiations, by their very nature, involve exchanges of things of value. Quid pro quos are not only legal; they are the goal of most such interactions. The response to this argument would be that the terms of these sorts of negotiations must involve the national interest, not the political (or financial) fortunes of the President. Another problem relates to the question of mixed motives. If Trump also wanted to withhold aid to Ukraine because he thought that other countries were not kicking in a fair share of support—which was, clearly, a legal motive on his part—would that negate his improper motive on the Biden dirt? The proof issues for prosecutors would be daunting.

In some ways, the legal setting surrounding President Trump’s possible impeachment represents a kind of mirror image of the backdrop to President Clinton’s impeachment, in 1998. There, the core accusation was that Clinton lied under oath about his relationship with Monica Lewinsky. Perjury is clearly a federal crime, but the question in Clinton’s case was whether his misconduct involved an abuse of Presidential powers. With Trump, his intervention in Ukraine appears to have been an abuse of his powers, but, conceivably, not a crime.

The debate about the criminality of the President’s behavior with regard to Ukraine, on some level, will always remain a theoretical matter. Under Department of Justice policy, sitting Presidents cannot be indicted; impeachment and removal must always come first. But the President and his supporters have already started making the argument that he should not be impeached because there is no proof of any underlying crime. The provisions of the Hobbs Act show that Trump may be wrong about that.