White House senior adviser Stephen Miller said Sunday that President Trump is prepared to shut down the government if Democrats don’t provide billions of dollars to build an expanded wall on the U.S.-Mexico border. Miller was speaking with Margaret Brennan of CBS’s “Face the Nation.”

Stephen Miller: “We’re going to do whatever is necessary to build the border wall to stop this ongoing crisis of illegal immigration. This is a”—

Margaret Brennan: “And that means a shutdown?”

Stephen Miller: “This is a very—if it comes to it, absolutely. This is a very fundamental issue. At stake is the question of whether or not the United States remains a sovereign country.”

A government shutdown could see more than 600,000 federal workers furloughed without pay just ahead of Christmas.