He declined re-election in 1851, and, in March 1851, was appointed by President Millard Fillmore, Minister to Brazil and also accredited to Uruguay, Argentine Confederation, and Paraguay. He was directed by the Government to visit Buenos Aires, Montevideo, and Asunción, and make treaties with the republics around the Río de la Plata and its tributaries. Several treaties were concluded with these governments by which the United States gained advantages never accorded to any European nation. The Democratic victory in 1852 caused the treaty of commerce with Uruguay to fail to be ratified by the United States Senate.