GM and Fiat Chrysler declined to comment, deferring to the Alliance for Automobile Manufacturers.
The American Automotive Policy Council, a different trade group that represents GM, Fiat Chrysler and Ford Motor Co., issued a strongly worded statement April 1.
"Any action that stops commerce at the border would be harmful to the U.S. economy, and in particular, the auto industry," said AAPC president Matt Blunt. "Access to Mexico’s market place and North American integration are critical to operations in the U.S."
The White House said April 1 that up to 2,000 inspectors who screen cargo and vehicles on the Mexican border may be reassigned to help handle the influx of migrants. That could slow the movement of goods even if trade isn't cut off completely.
Senior aides defended a border closing on March 31 news shows, with acting White House chief of staff Mick Mulvaney saying it would take “something dramatic” to avoid the closure.
Trump's threats to close the border are part of the president's longer-running effort to install a wall at the U.S.-Mexico border. After declaring a national emergency at the border in February, an attempt to access wall funding voted down by Congress, the president has said he would end aid to Guatemala, Honduras and El Salvador.
As the industry braces for a possible closure of the Mexican border, several other trade issues hang in the balance. A proposed replacement for the North American Free Trade Agreement---known as the United State-Mexico-Canada Agreement---is still up in the air. And an investigation into whether the Trump administration can levy tariffs on imported cars and parts in the name of national security still sits with the president, who has until May to act on the report.