The Mugwumps were Republican political activists in the United States who were intensely opposed to political corruption. They were never formally organized. Typically they switched parties from the Republican Party by supporting Democratic candidate Grover Cleveland in the presidential election of 1884. They switched because they rejected the long history of corruption associated with Republican candidate James G. Blaine. In a close election, the Mugwumps claimed they made the difference in New York State and swung the election to Cleveland. The jocular word "mugwump", noted as early as 1832, is from Algonquian mugquomp, "important person, kingpin" (from mugumquomp, "war leader"), implying that Mugwumps were "sanctimonious" or "holier-than-thou" in holding themselves aloof from party politics.

After the election, "mugwump" survived for more than a decade as an epithet for a party bolter in American politics. Many Mugwumps became Democrats or remained independents, and most continued to support reform well into the 20th century. During the Third Party System, party loyalty was in high regard, and independents were rare. Theodore Roosevelt stunned his upper-class New York City friends by supporting Blaine in 1884; by rejecting the Mugwumps, he kept alive his Republican Party leadership, clearing the way for his own political aspirations.

New England and the Northeast had been a stronghold of the Republican Party since the Civil War era, but the Mugwumps considered Blaine to be an untrustworthy and fraudulent candidate. Their idealism and reform sensibilities led them to oppose the corruption in the politics of the Gilded Age.