President Donald Trump has repeatedly threatened to “close the border” with Mexico — but what exactly does that mean?

The short answer is closing ports of entry into the U.S., as explained by acting White House Chief of Staff Mick Mulvaney.

Telling ABC’s “This Week” that Democrats won’t provide extra money or personnel for border security, Mulvaney said, “faced with those limitations, the president will do everything he can. If closing the ports of entry means that, that’s exactly what he intends to do.”

Trump said he’d close the border or “large sections” of it this week if Mexico didn’t stop all illegal immigration going north into the U.S. Monday, Trump repeated that threat.

Ports of entry along the southern border include El Paso, Texas, and San Diego, Calif.

Such an action by Trump would severely disrupt cross-border commerce if applied not just to people but to vehicles and trains that carry goods.

“Dozens of U.S. industries, starting with the auto industry, depend on just-in-time manufacturing, with parts arriving from Mexico,” Andrew Selee, president of the Migration Policy Institute, told MarketWatch in an email. “Slowing the border hurts both countries.”

If Trump follows through with his threat, he wouldn’t be the first president to close the border.

In 1985, President Ronald Reagan temporarily closed ports of entry along the southern border following the murder of a DEA agent in Mexico. Richard Nixon also closed it over a drug-related issue in 1969 and Lyndon B. Johnson sealed the border after the assassination of John F. Kennedy.