President Trump on Thursday raised the prospect of delaying the November election while making unsubstantiated claims that mail-in voting will lead to fraud.

There has been no evidence to back up Trump’s repeated claims about fraud in mail-in voting.

Trump has previously denied that he wanted to move the November election. In April, responding to Democratic rival Joe Biden’s suggestion that he would try to change the date, Trump said, “I never even thought of changing the date of the election. Why would I do that? November 3rd. It’s a good number. No, I look forward to that election and that was just made up propaganda.”

Even if he wanted to, though, the president has no power over when the United States holds federal elections.

If not the president, then who does?

Congress. Unlike some constitutional language that can be widely interpreted, the founders were unambiguous about how Election Day would be chosen: Congress is charged with choosing the date, and that date must be the same for the entire country.

Congress chose a date, the first Tuesday following the first Monday in November, in 1845, and it has never been changed.

But what about in an emergency like the one we’re in?

Even in an emergency, such as a global pandemic, the president can’t circumvent Congress and postpone or cancel the general election. And it’s extremely unlikely Congress would move it.

“In an emergency, the president is able to do a lot of things he normally could not do, but only because he has been designated these powers by Congress in laws such as the National Emergencies Act,” said Sylvia Albert, director of voting and elections at Common Cause, a nonprofit group that advocates for eased ballot access. “But in this case, the Constitution empowers Congress, not the president, to select Election Day. No laws passed by Congress have delegated these powers to the president, even in an emergency, so Congress is the only entity that has the power to change the date of the election.”

But haven’t lots of states changed primary election dates?

This is different from primary election dates, which are set by states governed by different rules. For general elections for federal offices, states are bound by federal law. Any effort by a state to unilaterally move or cancel the November election would be unlawful, and any results of a future election would be invalid, said Nicholas Stephanopoulos, professor at Harvard Law School.

Really, there’s nothing he could do?

Well, a president could try to lobby Congress to change the date. In a hypothetical scenario in which Trump wanted the election moved, he could publicly advocate for it and persuade Republicans to agree, but both chambers of Congress would need to vote to change the date. Even if Trump got his party behind him, the House is controlled by the Democrats, who would be unlikely to take the unprecedented step of postponing the general election at Trump’s behest.

Is there anything else the president could do to affect Election Day?

Rick Hasen, professor of law and political science for the University of California at Irvine’s law school, said there are other ways that the president could change how the election works without actually moving the date.

If the coronavirus is still a factor in the fall, Trump could claim emergency powers to keep people in cities where outbreaks have been worse from going to polling places in person, in the name of public health, Hasen said. That could depress turnout in the heavily populated urban areas that tend to vote Democratic.

Former vice president Joe Biden, Trump’s presumptive challenger this fall, in April warned supporters that Trump may try to move the election and also mentioned his moves to put conditions on a loan for the U.S. Postal Service, which is struggling amid the coronavirus crisis. Biden claimed Trump was trying to make it harder for people to vote by mail, which he has attacked before. “That’s the only way he thinks he can possibly win,” Biden said.

Election officials in both parties, as well as many Democrats, are trying to expand vote-by-mail in response to growing concerns that voters won’t be comfortable going to polling places this fall because of the coronavirus. Many Republicans, including Trump, have rejected these efforts.

Hasen said the president could also lean on state legislatures to take voting for president away from citizens entirely in the name of public safety. The members of the electoral college who officially choose the president do not need to be chosen by the voting public. Instead, the state legislatures could select the presidential electors as they did in the early days of our republic.

This would be problematic for Democrats, Hasen said, if Republican-held legislatures in swing states went that path.

But it’s not that likely.

“That would be constitutional,” Hasen said, “but I believe it would provoke massive social unrest.”

Donna Cassata contributed to this report.