WASHINGTON — A Republican lawmaker plans to introduce legislation on Thursday that would embolden President Trump’s trade war by granting him sweeping new executive powers to increase tariffs on imports.

The proposal, by Representative Sean P. Duffy of Wisconsin, is unlikely to pass Congress, but it has alarmed conservative advocacy groups and business lobbyists, who warn that such an idea could further destabilize the global rules of trade and prompt other countries to adopt similar presidential powers.

The legislation would allow Mr. Trump to unilaterally raise tariffs on a foreign product if he determined that a trading partner had imposed “significantly” higher tariffs or other trade barriers on that same American good. The draft proposal would also give the president power to enter into new trade agreements without congressional approval.

The president already has broad powers to impose tariffs but must go through certain channels to put them into effect, including a formal investigation by the Commerce Department or other agencies. His actions can also be overturned, in some cases, by the United States International Trade Commission or the World Trade Organization.