Complex sentence: Abel had served as secretary-treasurer since 1952 and been a longtime McDonald supporter, but Abel strongly criticized the Human Relations Committees and promised a return to militancy to win higher wages and benefits for workers.
Split into simpler sentences: Abel had served as secretary-treasurer since 1952, and was a long-time McDonald supporter. But Abel strongly criticized the Human Relations Committees, promised a return to militancy in order to win higher wages and benefits for workers.