President Donald Trump has alluded to arcane legal arguments in repeatedly suggesting he could seek a third term. Besides challenging long-settled readings of the U.S. Constitution, a Trump move to run in 2028 would challenge the precedent that voters have repeatedly upheld when given the opportunity.

Here is an explanation of the historical and legal tradition behind the presidency being a job for a maximum of two terms and two terms only.

Washington set the example of voluntary limits

It seemed a foregone conclusion that Washington, president of the 1787 convention that yielded the U.S. Constitution, also would become the nation’s first federal executive, even as anti-federalists worried that he’d be reelected again and again, becoming a quasi-king by acclamation.

Washington began his presidency in 1789, leading an executive branch of government that the Constitution’s authors balanced with two others: Congress and the judiciary.

Besides those structural guards against concentrations of power, Washington put aside his military garb and title, opting for the era’s formal attire and the honorific of “Mr. President” to underscore his status as an elected civilian. He considered not standing for reelection. He even had James Madison draft a farewell address before ultimately seeking and winning another term in 1792. Four years later, he tasked Alexander Hamilton with dusting off and polishing up Madison’s farewell draft as he announced his retirement from public life.

There was no legal barrier to a third Washington term. But his decision set the tone. Four of the next six presidents won a second term but passed on a third. The last of that group, Andrew Jackson, was the first president not to have worked with Washington or have known him. Yet by the time Jackson endorsed his own successor, Martin Van Buren, two terms had become the standard.

A few pushed against the Washington rule – and failed

Historians have debated whether Abraham Lincoln might have pursued a third term after the Civil War had he not been assassinated in 1865 at the outset of his second term.

Ulysses S. Grant, Lincoln’s victorious Civil War general and president from 1869 to 1877, led Republican delegates’ initial voting at their 1880 convention. But he could not win a majority.

Theodore Roosevelt, elected vice president in 1901, served nearly a full presidential term after William McKinley’s death in 1901. When Roosevelt was elected in his own right in 1904, he promised he would not run for what he called a third term.

Delegates at the 1908 GOP convention chanted “four more years,” but Roosevelt kept his word. He backtracked in 1912 but lost the nomination to his successor, incumbent William Howard Taft. Roosevelt launched a failed third-party campaign and lost, pilloried by critics for his broken third-term promise. One scathing political cartoon depicted the ghost of George Washington chiding Roosevelt.

FDR used World War II to win additional terms

In 1940, Franklin Delano Roosevelt became the only president to successfully win a third election, doing so as World War II raged in Europe ahead of the eventual U.S. entry.

Biographer H.W. Brands reasoned that FDR saw the global conflict as a “chance to write his name in bold letters across the history of the world.” But the 32nd president carefully couched his decision as one of necessity, not ambition. “Precisely when he determined to try for a third term is unclear,” Brands wrote. “He never revealed his thinking on the subject.”

Roosevelt sidestepped reporters’ questions about his plans in 1940. At that year’s Democratic convention, his ally Sen. Alben Barkley of Kentucky told delegates, with FDR’s blessing: “The president has never had and has not today any desire or purpose to continue in the office. … He wishes in all earnestness and sincerity to make it clear that all delegates to this convention are free to vote for any candidate.”

But at the same time, and also with FDR’s blessing, Chicago Mayor Ed Kelly was working delegates for the president. After securing a third nomination that had eluded Grant and his distant cousin, Teddy Roosevelt, FDR accepted it in a radio address:

“I have had to admit to myself, and now to state to you, that my conscience will not let me turn my back upon a call to service,” he said. “The right to make that call rests with the people through the American method of a free election. Only the people themselves can draft a president.”

Voters reelected Roosevelt twice more – but decided never again

FDR won two more terms, though not without critics. His first vice president, John Nance Garner, sought the 1940 nomination in Chicago. Some Capitol Hill allies quietly grumbled as well about a figure they saw as holding power too tightly.

And while Roosevelt won Electoral College landslides in each of his four victories, his share of the popular vote dwindled from his 60.8% peak in 1936 to 54.7% in 1940 and 53.4% in 1944.

Roosevelt died in April 1945. Vice President Harry Truman replaced him.

Not long after Roosevelt’s death, Congress began earnest consideration of what became the 22nd Amendment, limiting presidents to two elections. Without naming Truman, lawmakers exempted the president serving at the time while also carving out a narrow way for someone to serve more than eight years: Someone who ascends from the vice presidency for less than half of one term could still win and serve two full terms of their own.

Truman, who served nearly all of FDR’s last term plus his own, did not immediately swear off another term in 1952. But in a stinging defeat for a sitting president, he lost the New Hampshire primary — and quickly declared would not seek another term.

Every future president has been bound by the 22nd Amendment

Lyndon Johnson met a similar fate 16 years later. Because he served less than half of the slain John F. Kennedy’s term, Johnson was still eligible to be elected twice. He won a 1964 landslide for a full term. But the Vietnam War chipped away at his popularity thereafter.

Johnson performed poorly in the New Hampshire primary on March 12, 1968. On March 31, he told a national television audience, “I shall not seek, and I will not accept, the nomination of my party for another term as your president.”

There has been occasional talk of repealing the 22nd Amendment since.

President Ronald Reagan, another two-term president, publicly supported repeal, telling an interviewer, according to The New York Times, that he “wouldn’t do that for myself, but for presidents from here on.”

Trump, on the other hand, makes clear that any changes in law or tradition would be for his benefit.

“I’m not joking,” he told NBC News. “There are methods which you could do it.”