A US trade body is due to make its final ruling on the Bombardier aircraft imports dispute.
The Trump administration has threatened to impose duties of 292%.
Unions have called on the US International Trade Commission (ITC) to set aside that decision when it meets on Friday to discuss whether rival manufacturer Boeing has suffered injury or loss from Atlanta-based Delta Airlines’ order of Bombardier’s C Series passenger jets.
If the ITC finds in Boeing’s favour, the tariffs will be applied, undermining thousands of jobs in Belfast where the C Series wings are produced.
The ITC’s role is to determine whether the aircraft manufacture industry in the US is damaged by imports that the administration believes are being sold too cheaply.
If it concurs, the Secretary of Commerce will order tariffs, which will be enforced by the US Customs Service.
Bombardier has received large sums from government administrations in the UK and Canada as part of the development of the C Series.
It has argued the Commerce Department’s tariff threat ignores long-standing business practices in the aerospace industry, including launch pricing and the financing of multi-billion dollar aircraft programmes.
Boeing has said its business was damaged because Bombardier received illegal government subsidies, dumping the C Series in the US through the cut-price 2016 Delta sale of 75 jets.
Prime Minister Theresa May has held talks with President Donald Trump at the World Economic Forum in the Swiss ski resort of Davos.
The two leaders began by discussing Bombardier, with Mrs May reiterating the importance of the company’s jobs in Northern Ireland.
Union leaders in Belfast argue she has not done enough to fight their case.
A Bombardier and Airbus partnership will include the construction of a new US manufacturing facility in Alabama for the assembly of the C Series.