Complex sentence: Hanna had been elected chairman of the RNC during the convention; he appointed an executive committee to run the campaign, initially selecting no New Yorker, not wanting to offend Platt or the McKinley supporters in New York who had opposed that boss.
Split into simpler sentences: Hanna had been elected chairman of the RNC during the convention; he appointed an executive committee to run the campaign. Initially, Hanna selected no New Yorker, not wanting to offend either the Platt faction nor the McKinley supporters in New York who had opposed that boss.