President Trump is set to unveil his most expansive tariffs to date on Wednesday afternoon, when he will detail potentially punishing levies on countries around the globe, including America’s largest trading partners.

Mr. Trump has promised for months to impose what he calls “reciprocal” tariffs, which the president says will correct years of “unfair” trade in which other countries have been “ripping off” America.

“We helped everybody, and they don’t help us,” Mr. Trump said on Monday.

Exactly how he plans to structure the new tariffs is not yet clear. The White House press secretary said Tuesday afternoon that Mr. Trump had decided on a course of action and that the new tariffs would go into effect immediately, but that he and his trade advisers were continuing to hash out details.

The president has talked about basing a new tariff rate for countries on the tariffs they place on American products, as well as other trading practices that the Trump team deems unfair.

Mr. Trump has also considered a flat 20 percent tariff on all trading partners. Such a levy would be aimed more at generating revenue to offset the tax cuts that he hopes to push through Congress.

Either approach would be a significant escalation toward a trade war that Mr. Trump seems eager to unleash. Governments across the world have been preparing to hit back if the president raises tariffs, raising the potential for a destabilizing economic battle that drives up costs as Mr. Trump tries to force supply chains back to the United States.

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