The travel industry is on edge after Homeland Security Secretary Markwayne Mullin reiterated his threat to withdraw U.S. Customs and Border Protection officers from airports in so-called “sanctuary cities” in a move that could jeopardize international flights.

The U.S. Travel Association said that Mullin confirmed he is considering withdrawing CPB officers in a meeting where the trade group was pressing its concerns about other proposals the Trump administration is considering that could hamper travel. U.S. Travel and the major airlines quickly condemned the idea, and even Transportation Secretary Sean Duffy said it doesn’t make sense to him.

“U.S. Travel believes such a move would have devastating consequences for the travel industry and communities that depend on international visitation,” the industry group said Friday in a statement.

Details of the meeting were first reported by The Atlantic.

Duffy told a Congressional hearing earlier this week that he wasn’t familiar with Mullin’s remarks, and he’d like to learn more about the context and maybe ask Mullin a question about what he meant. But Duffy said it would be a bad idea to start restricting travel based on political views. After all, at some point Democrats will be in charge and “you will all switch spots at one point — hopefully not too soon Mr. Chairman,” Duffy said.

“We have people from around the world and around the country that need to be able to fly into all different kinds of places. We shouldn’t shut down air travel in a state that doesn’t agree with our politics,” Duffy said.

So it’s not clear how much support this idea has within the administration, though President Donald Trump has previously threatened to withhold funding from sanctuary cities.

There is no strict definition for sanctuary policies or sanctuary cities, but the terms generally describe limited cooperation with Immigration and Customs Enforcement. And courts have rejected the idea of pulling funding from them in the past.

In Trump’s first term in office, in 2017, courts struck down his effort to cut funding to the cities.

It’s not clear exactly which cities and airports Mullin might target, but the Justice Department last year published a list of three dozen states, cities and counties that it considers to be sanctuary jurisdictions.

The Airlines for America trade group was quick to say the idea would hurt the economy and disrupt travel.

“Reducing CBP staffing at major airports would have a devastating effect on the airline and tourism industries, causing a significant operational disruption to carriers, travelers and the flow of international cargo.”

Josh Funk And Rio Yamat, The Associated Press