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Which rulers and groups supported Pope Alexander III against Frederick I?

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Medieval manuscript miniature showing Frederick I as a crowned crusader holding an orb and scepter, with Henry of Schaftlarn kneeling and presenting a book to him. — Frederick I Frederick I as a Crusader with (right) Henry of Schäftlarn dedicating to him a copy of the *History of the First Crusade* by Robert of St. Remy; miniature from a manuscript in the Vatican Library.

Pope Alexander III was supported by a wide range of European powers and local Italian groups who opposed the efforts of Holy Roman emperor Frederick I to consolidate imperial authority.123

Key supporters included:

European Monarchs: France, England, Spain, and Hungary recognized Alexander III as the true pope. Additionally, the Byzantine emperor Manuel I Comnenus prepared to form an alliance with France and was ready to recognize Alexander.2
The Normans of Sicily: The Kingdom of Sicily, under William I, was one of the first to recognize Alexander III as the legitimate pope.13
The Lombard League: This league of northern Italian cities, including Milan, Venice, Mantua, Padua, Brescia, and Lodi, formed specifically to defend their independence against Frederick I. They provided the Pope with essential military and political support, eventually forcing the Emperor to acknowledge him in the Peace of Venice.423
The Clergy: Alexander maintained the loyalty of most of the clergy in Italy and many within Germany throughout the schism.3