On Thursday morning, Trump officially called for delaying the November 3 election. He claimed that increased mail-in voting due to the coronavirus health crisis would lead to fraud, although there is little to no evidence that the two are connected.

“With Universal Mail-In Voting (not Absentee Voting, which is good), 2020 will be the most INACCURATE & FRAUDULENT Election in history. It will be a great embarrassment to the USA. Delay the Election until people can properly, securely and safely vote???” the president tweeted.

The statement immediately prompted a backlash from lawmakers and legal experts.

“It is unprecedented for one of the presidential candidates to talk about delaying an election,” Timothy Naftali, a historian at New York University and a former director of the Richard Nixon Presidential Library and Museum, told Newsweek. “There is no provision in the Constitution for delaying the election. You can’t do this under our system of government.”

The number of presidential elections that have been formally disputed—meaning there was an intervention from Congress or the Supreme Court to decide the outcome—can be counted on one hand.

The most recent was in 2000 when the Supreme Court stepped in to help decide the race between George W. Bush and Al Gore. While Gore won the popular vote, the contest came down to an Electoral College battle in Florida. Florida officials were in the process of recounting ballots when the Court effectively awarded Florida’s 25 electoral votes to Bush.

The next trio of disputed elections all happened in the nineteenth century.

In 1800, the House of Representatives had to decide who the vice president would be after Thomas Jefferson and Aaron Burr tied at 73 electoral ballots each. At that time, Electoral College delegates voted for two candidates: the top finisher became president and the runner-up became vice president. The process was amended in 1804 when the 12th Amendment was passed, which required that electors vote specifically for a president and vice president.

Twenty-four years later, the House again had to determine official results, but this time for a presidential election. While Andrew Jackson won more electoral votes, he didn’t receive a majority, which prompted Congress to choose between him and John Quincy Adams. What happened next was what Jackson supporters deemed a “corrupt bargain”—then-Speaker Henry Clay forged a coalition that secured the White House for Adams and later he became Adams’ secretary of state.

In 1876, Republican Rutherford B. Hayes had lost the popular vote to Democrat Samuel Tilden. But three states had disputed results, which caused the two parties to make a deal: Democrats gave Hayes the presidency in exchange for Republicans removing all federal troops from the South, ending the Reconstruction era.

There have been multiple other instances of alleged voter fraud in presidential elections but they didn’t result in formal action. The extremely close contest between Richard Nixon and John F. Kennedy in 1960 is a recent example. Republican officials cried foul and stated that voting machines were rigged, but Nixon never contested the results.

“The common factors [for disputed elections] include intense political partisanship, voter intimidation, old-fashioned election administration errors and, on occasion, outright fraud. The fraud part is rare,” Brandon Rottinghaus, a political science professor at the University of Houston, told Newsweek.

Rottinghaus added that despite Trump’s many tirades against mail-in balloting, election fraud is “not common and it’s certainly not as bad as it has been in past elections.”

A majority of Americans have said they would find it inappropriate if the November election’s outcome was disputed in a variety of ways, according to a report released last month by the Democracy Fund Voter Study Group. About two-thirds of Americans, or 67 percent, said they believed it would be inappropriate if Trump loses but refuses to leave office because he claims he has credible evidence of illegal voting.

“There is something fishy about a candidate alleging voter fraud before the election has even started,” Naftali said.