Sen. John McCain said Friday that Mexico paying for President Donald Trump’s oft-promised border wall is “not a viable option.”

When CNN’s Manu Raju asked the Arizona Republican whether it is a “viable option” for Mexico to pay for a border wall, McCain responded simply, “No.”

Asked why he said that, McCain offered, “Because it’s not a viable option.”

Trump continues to insist that Mexico will pay for his proposed border wall, despite the fact that Mexican leaders have made clear that they have no intention of doing so. McCain, one of Trump’s most vocal Republican critics in Congress, said Friday that he does not agree with the president on this front, either.

In the interview, McCain also voiced broader skepticism of the proposal, which critics — both conservative and liberal — have argued would be extraordinarily expensive but do little to solve problems along the border.

“If you only build a wall … without using technology, individuals, drones, observation, et cetera, you’re not going to secure the border,” McCain said.

CNN also asked McCain if taxpayers would be left to foot the bill for the wall if it’s built. He said in response: “Taxpayers are paying a lot of money right now. One of the biggest problems we have is the enforcement of existing law.”