WASHINGTON—The head of a major firefighters union, a supporter of Democratic Party causes and a confidant of presidential candidate Joe Biden, is mired in an internal dispute over allegations of financial mismanagement, improper pension payouts and misuse of union resources for personal expenses.

Harold Schaitberger, the longtime president of the International Association of Fire Fighters, is accused by the union’s treasurer of prematurely collecting well over $1 million in payments from a union pension fund while still working…

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