The International Trade Administration, a part of the Commerce Department overseen by Mr. Kaplan, the former Zekelman lawyer and lobbyist, sided with Zekelman, recommending that Borusan’s request be denied, documents show. A spokeswoman said Mr. Kaplan had honored all ethics requirements, which would prohibit him from helping a recent client.

Mr. Trump announced last summer that he would double tariffs on steel from Turkey to 50 percent. Those imports — including steel tube — dropped 48 percent last year as a result, a drop so big that Mr. Trump moved last week to return the tariff to 25 percent.

Mr. Zekelman said he played no role in asking for the temporary doubling of the tariff on Turkey, but each of these moves helped his business. The company is also making structural steel pipe for use in the parts of the border wall Mr. Trump intends to build with emergency government funds.

On Friday, the administration reached a deal to eliminate the 25 percent tariffs on steel imports from Canada and Mexico, while leaving them in place for most of the rest of the world. The decision will benefit Mr. Zekelman, whose biggest manufacturing plant is in Harrow, Ontario, meaning the company will now be able to ship Canadian-made pipe into the United States tariff free, even as Mr. Zekelman advocates protection for his United States plants.

The success of his tactics has not gone unnoticed by competitors. The American-Turkish Council, whose sponsors include Borusan Mannesmann, moved its annual meeting from the Ritz-Carlton hotel to the Trump International Hotel in Washington. Among the guests at the event last month were Mr. Ross, the commerce secretary.

“What else are you going to do?” said Ms. Mendoza, the lawyer who has represented Borusan. “Everybody is trying to play the game.”