Washington (CNN) President Donald Trump, who had been pressing staff to quickly finalize his promised tariffs on steel and aluminum, plans to sign the order on Thursday at the White House, three officials familiar with the matter said.

White House aides are planning for an announcement at 3:30 p.m. ET in the Roosevelt Room, the three officials said, with steel and aluminum workers being flown to Washington for the event.

Trump had applied pressure on his staff to complete the tariffs before the end of this week, hoping the announcement can help him rally voters behind the Republican candidate in a Pennsylvania special election. The precise timing of the announcement remained in flux for much of Wednesday as administration officials rushed to complete the trade language.

On Capitol Hill and in the West Wing, efforts are underway to sort out which countries may be excluded from the final plan, which Trump announced hastily during a meeting with industry executives last week. The potential tariffs have caused deep anxiety among US trading partners and Republican leaders in Congress.

"There are potential carve-outs for Canada and Mexico based on national security, and possibly other countries as well," press secretary Sarah Sanders told reporters on potential exemptions from the tariffs. Sanders said the national security exemption would be on a "case-by-case" and "country-by-country" basis.