European Union trade officials have warned such a move would result in counter-tarrifs being slapped on exports from the US within days.
They said iconic US products ranging from Harley-Davidson motorcycles to Jack Daniels whiskey would be targeted in any retaliatory strikes and that preparations for such a confrontation were in full swing.
Analysts said the choice of products was significant as Harley-Davidson is based in Wisconsin, home state of Republican Speaker Paul Ryan, and Bourbon is mainly produced in Tennessee and Kentucky, home of Donald Trump supporter Mitch McConnell, who is the Senate Majority Leader.
Officials in Brussels insist the list dates back to the last major commercial dispute when George W Bush was in the White House but acknowledged this had been recently adapted.
The EU’s willingness and capability to react “immediately and adequately” in such a situation was made clear by European Commission President Jean-Claude Juncker last summer.
A defensive backlash is allowed under World Trade Organisation (WTO) regulations although Brussels said it would also lodge an official complaint with officials from the global body.
Steel and aluminium are particularly touchy subjects between the US, China and the EU because they are so closely linked to global over-capacity and prices are constantly under heavy pressure.
The final decision on import tariffs lies with Mr Trump.
Unlike tax laws, which are ultimately decided by Congress, the President has a free rein with customs.
United States Secretary of Commerce Wilbur Ross is trying to justify the plans with a WTO rule that allows trade restrictions on grounds of “national security.”
The US likes to make use of this exception although it is actually meant times of armed conflict rather than trade wars.
(Additional reporting by Monika Pallenberg)