In January 1881 he was elected with the support of the Stalwart faction to represent New York in the United States Senate. He became a member of the Forty-seventh Congress and the chairman of the Committee on Enrolled Bills. However, he served only from March 4 to May 16, 1881, when he and Conkling resigned because of a disagreement with President James Garfield over federal appointments in New York. (Platt resigned at Conkling's insistence, earning him the nickname of "Me Too" Platt.) The immediate occasion of their resignation was Garfield's appointment of Half-Breed faction leader William H. Robertson as Collector of the Port of New York. Soon afterward, however, Garfield's assassination by Charles J. Guiteau, a self-proclaimed Stalwart who claimed friendships with Platt and Conkling, was the finishing blow for their faction. Platt and Conkling ran in the special election to fill the vacancies created by their own resignations but lost. Eschewing elective office, Platt then devoted his attention to mending fences and rebuilding the machine, which he then ran after 1887 as an "easy boss."