Initial use of term was had cynical relation to Natick tribe (Algonquian dialect) 'mugquomp' which meant 'great man.' Pre-1884 use of 'mugwump' meant 'bigshot' and was used to ridicule post-Civil War patrician politicians. Term became synonymous with 'traitor' in 1884 when it was used to identify Republicans who supported Democrat Grover Cleveland over their party's nominee, James Blaine (Cleveland won popular vote by .3% and electoral college by 37 votes). Word then metamorphosed into above use, perhaps because Mugwumps advocated civil service reform that ostensibly transcended partisan politics in interest of public good (as defined by Mugwumps, of course).
The mugwumps themselves were an aristocratic clique who resented that systems of patronage and machine politics had displaced them. They had contempt for the 'masses' and their activities generally consisted of membership in exclusive and insular organizations and writing scholarly articles. Ideologically, they promoted a mix of christian morality, republican virtue, and classical liberalism. Michael Hoover