Rosewater served on the Republican National Committee during the late 19th century. During 1888 he built the Bee Building, a downtown landmark which was demolished during 1966. During 1897, at the behest of President McKinley, Rosewater came to Washington D. C. to direct the U. S. delegation at the Congress of the Universal Postal Union (the international body responsible for promoting efficiency in the flow of mail from country to country, tasked that year with securing cheaper international postage). This experience influenced his work as an organizer for the 1898 Trans-Mississippi Exposition, for he prevailed on the Post Office to produce a special Trans-Mississippi Issue of nine stamps commemorating the Exposition, and was credited with much of the success of that event. The most profitable event of the exposition, an Indian Congress that convened representatives of some 35 tribes was "the child of [Rosewater’s] brain," according to the Congress's chief ethnological consultant James Mooney, and its "successful outcome was due chiefly to his tireless activity and unfaltering courage." Rosewater also ran two losing campaigns for a U.S. Senate seat in Nebraska. He died at the "Omaha Bee" building on August 30, 1906.