Trump tore into the EU in a CBS News interview over the weekend, calling the union a "foe" of Washington because of its trade policies, which the president has long decried as one-sided and unfair to the U.S.
The department opened an investigation in May into whether imported autos and parts pose a national security risk and was holding a hearing on the probe on Thursday, taking testimony from auto trade groups, foreign governments and others.
Toyota North America Chief Executive Jim Lentz told company dealers in an email seen by Reuters that "Toyota and the entire auto industry is facing a very unsafe public policy threat - a potential 25% tariff on all imported autos and auto parts - and we need YOU to help us win this battle!"
Trump has alienated some allies over his protectionist trade policies, including tariffs on European Union aluminum and steel exports to the U.S.
The Alliance of Automobile Manufacturers, whose members include General Motors Co, Volkswagen AG and Toyota Motor Corp, said it plans to issue the warning at a public hearing on Thursday. She says security, counterterrorism, and energy will be on the agenda, as well as trade.
They are also facing higher prices after tariffs were imposed on aluminum and steel.
A study by the Center for Automotive Research also released Thursday estimated auto demand will fall by between 493,600 and two million vehicles, resulting in the loss of 82,000 to almost 750,000 auto manufacturing jobs.
There are also deep fears US levies would harm the overall Canadian and American economies because collateral damage would reach well beyond the critical, thriving auto sector.
No automaker or parts company has endorsed the tariffs, and they have pointed to near-record sales in recent years.
Juncker has been outspoken in his criticism of Trump's decision to impose trade tariffs and last month told lawmakers that Trump had called him a "brutal killer" at a Group of Seven meeting in Canada. "And you know what we're talking about with respect to cars and tariffs on cars".
European Commission chief Jean-Claude Juncker said United States efforts to split the EU on trade "are in vain", in a message Wednesday (Jul 18) to President Donald Trump before their talks in Washington next week.