Do federal conflict-of-interest laws apply to the president? Do the criminal laws that prohibit officials from participating in any decision in which they have a financial interest apply to the man or woman in the Oval Office?

The prevailing assumption, even on the part of those who have explored the multiple entanglements between Donald J. Trump’s financial holdings and the innumerable ways in which presidential action could benefit them, is that conflict-of-interest laws don't apply to the man or woman at the very top. It's anathema to think that any law other than the Constitution can set eligibility requirements for the presidency. As George W. Bush's ethics adviser, Richard Painter, has said, "Constitutionally it's going to be very hard to prohibit anyone from becoming president by making them divest of their holdings."

But in fact it's not clear that the conflict-of-interest laws don't apply to the president. Those who think they don't might have in mind a 1983 U.S. Office of Government Ethics (OGE) opinion, which expressed the view that the conflict-of-interest laws strictly speaking do not extend to the president—although OGE went on to say that "as a matter of policy, the President and the Vice President should conduct themselves as if they were so bound." And every president since the passage of the 1978 Ethics in Government Act has done so. Every president has agreed to put his assets in a blind trust, where they are sold off and replaced by financial instruments whose identity remained unknown to him.

But why did the OGE stop just short of identifying a legal requirement on the president? It was relying on a 1974 Justice Department opinion written by then acting-attorney general (later Judge) Laurence Silberman. But that opinion—and it's only an opinion, not law—was based on an extraordinarily weak argument.

Silberman noted that a 1960 Bar Association report written to aid Congress in crafting conflict-of-interest laws had questioned whether they could apply to the president. Why? The only specific reason the report gave is that a president receives so many gifts from all over the world that he couldn't possibly return them all. Silberman, noting that Congress never expressly disavowed the report's particular observation on this narrow matter of gifts, then concluded that Congress must have sweepingly intended that the law not apply to the president at all, even in cases of vast and inveterate conflicts of interest. That's a stretch, to say the least. The better interpretation is that Congress expressed no clear intent on the matter of whether the law applies to the president.

What about the Constitution? Article II says, "The President shall, at stated Times, receive for his Services, a Compensation, which shall neither be encreased nor diminished during the Period for which he shall have been elected, and he shall not receive within that Period any other Emolument from the United States, or any of them."

That's a conflict-of-interest provision. True, it was designed to prevent a president from conflicts of interest that arise when other branches of government, or state governments—not private companies—might be paying him money. But that constitutional provision shows that a president's sources of financial remuneration can disqualify him from office, and do so precisely because they place him in a conflict of interest. Today's criminal laws that prohibit officeholders from holding certain kinds of private interests can't, then, be deemed inapplicable to the president, simply on the grounds that it's illegitimate to "prohibit anyone from becoming president by making them divest of their holdings." The Constitution does just that.

Trump himself has recognized that as president he would have to take some action to remedy his conflicts of interest. The remedy he has in mind is to have his children manage his holdings. But that is not a remedy for conflict of interest as understood by the law and regulations. An officeholder is deemed to be in conflict every bit as much because of his children's interests as his own. And in any case, while his children might be managing his companies, Trump would still hold his interests in them. And so he would have done exactly nothing to address what he himself acknowledges is a serious conflict of interest.

It's not clear, then, that the conflict-of-interest laws don't apply to the president. Nor is it true that constitutional principle would necessarily bar their application. The law, never having been tested on that question in court, remains uncertain.

Donald Trump, more than anyone, should realize what a problem that is. After all, as he has said, it would be a tragedy if Americans elected a president, Ted Cruz, who couldn't serve because a court deemed him ineligible for not being a natural-born American. And it's unclear, Trump says, whether Cruz—having been born to an American mother but in Canada—is a natural-born American, since the law has never been tested on that question. Someone, eventually, would take a President Cruz to court on the matter, possibly plunging the nation—so Trump warns—into a governance and constitutional crisis.

A similar risk exists for Trump, who faces his own possibly calamitously disqualifying legal unclarity, although in his case the threat would come from a criminal prosecution that would test the meaning of the conflict-of-interest laws, not a constitutional suit that would test the meaning of "natural-born American." By Trump's own lights, that would place the country in an untenable position. And so, again by his own lights, he faces two choices. He can commit to having his vast holdings immediately sold off and his assets managed through a blind trust if he is elected president. Or he can drop out of the race.

Andrew Stark, a professor at the University of Toronto, is the author of Conflict of Interest in American Public Life (Harvard University Press, 2000).