Mugwumps were Republicans who supported Democratic presidential candidate Grover Cleveland in 1884 because they viewed their own party’s candidate, James G. Blaine, as corrupt. Many historians believe the Mugwumps swung the election to Cleveland by helping him win in New York and its 36 electoral votes.
After the election, the term came to mean someone who is independent or who remains undecided or neutral in politics.
Michael Quinion: “It hit the big time in 1884, during the presidential election that set Grover Cleveland against the Republican James G Blaine. Some Republicans refused to support Blaine, changed sides, and the New York Sun labeled them little mugwumps. Almost overnight, the sense of the word changed to turncoat. Later, it came to mean a politician who either could not or would not make up his mind on some important issue, or who refused to take a stand when he was expected to do so. Hence the old joke that a mugwump is a person sitting on the fence, with his mug on one side and his wump on the other.”
The Atlantic notes their opponents also sneered at Mugwumps as “hermaphrodites.”