In a letter sent Friday afternoon, Committee Chairman Eliot L. Engel demanded that the ambassador, Todd Chapman, produce “any and all documents referring or related to any discussions” he has held with Brazilian officials in recent weeks about their nation’s tariffs on ethanol, an important agricultural export for Iowa, a potential swing state in the American presidential election.

The committee’s letter was sent in response to reports in the Brazilian news media this week saying that Mr. Chapman, a career diplomat, made it clear to Brazilian officials they could bolster Mr. Trump’s electoral chances in Iowa if Brazil lifted its ethanol tariffs.

Eliminating tariffs would give the Trump administration a welcome trade victory to present to struggling ethanol producers in Iowa, where the president is in a close race with his Democratic rival, Joseph R. Biden Jr.