President Donald Trump has been back in the White House for just more than 100 days, and he’s already thinking about a third term. For much of American history, the notion would have been laughable.

Nearly a century ago, the historian John Bach McMaster surveyed the first 138 years of the presidency and hazarded a prediction in the pages of The Atlantic: “Should the time come when a president who has twice been elected to office seeks a third election, he will surely meet great opposition, for the no-third-term doctrine is still strong.”

Within 13 years, he would be proven wrong. In 1940, Franklin D. Roosevelt coasted to an unprecedented third term, capturing 55 percent of the popular vote and a whopping 85 percent in the Electoral College. As the writer Gerald W. Johnson observed the following year, 27 million voters “trampled down the thitherto sacred third-term tradition in order to reëlect the chief New Dealer.”

Roosevelt was breaking no law at the time he sought a third term. The two-term presidential limit was a mere custom established when George Washington stepped down voluntarily after eight years in office. Two presidents—Ulysses S. Grant in 1880, and FDR’s fifth cousin Theodore Roosevelt in 1912—had previously tried (and failed) to return to the White House for third, nonconsecutive terms. Roosevelt’s victory was not a surprise, and certainly not to readers of this magazine at the time. Barely a year into FDR’s second term, the journalist J. Frederick Essary made a prediction that would hold up much better than McMaster’s: “If Mr. Roosevelt runs a third time,” Essary wrote, “he will be renominated and reëlected.”

But no president would do so again. Roosevelt won a fourth term in 1944, as the nation chose not to replace its commander in chief during the height of World War II. The president’s worsening health was unknown to the public, and he died less than three months after his fourth inauguration, in April 1945. His death, and the end of the war soon after, revived a debate over whether to formalize what McMaster called “the unwritten law of the Republic.” America’s founders had considered writing a term limit into the original Constitution as a way to prevent a power-hungry president from becoming too much like a king. After Roosevelt’s death in office, and after having just fought a war to defeat dictators in Europe, that argument gained new momentum. In 1951, the states ratified the Twenty-Second Amendment, which says that “no person shall be elected to the office of the President more than twice.”

Such an ironclad prohibition would seem to rule out a third term for President Donald Trump. But that hasn’t stopped him or his biggest supporters from musing about the possibility of another run in 2028. “I’m not joking,” he told NBC News last month. “There are methods which you could do it.” (As if to prove the point, or to troll his critics, the official retail website of Trump’s company is now selling Trump 2028 hats.) When my colleague Ashley Parker asked Trump about a possible third term last week, he said it was “not something that I’m looking to do.” But he was clearly intrigued by the idea: “That would be a big shattering, wouldn’t it?”

To get around the Twenty-Second Amendment, Trump’s allies have floated the idea that he could run for vice president on the ticket of, say, J. D. Vance in the next election. If Vance won, he could resign, thereby making Trump president without him having to be “elected” to the office more than twice. (The Twelfth Amendment, however, seems to cut off that path, because it states that “no person constitutionally ineligible to the office of President shall be eligible to that of Vice-President of the United States.”) Or Trump could simply run for president and dare the Supreme Court to throw him off the ballot in the middle of an election.

Should the Supreme Court blink, the decision of whether two terms of Trump are enough would fall to voters. The president has never been as popular as FDR was during his years in the White House. But if history is a guide, it would be wrong to assume the public would automatically uphold a long-established limit. Just ask Essary: “It is difficult to believe that the mass of the people care very deeply about the third term in itself,” he wrote in 1937. “There is nothing in our experience as a nation to prove that they do care; and there is much to indicate how little the average man concerns himself about the matter.” It’s a sentiment that, some nine decades later, Trump might be willing to bet on.