Complex sentence: He was then elected as chairman of the Republican State Committee, serving on several victorious campaigns before, in 1896, being chosen as chairman of the Massachusetts Republican delegation to the St. Louis Convention, which nominated William McKinley for president.
Split into simpler sentences: He was then elected as chairman of the Republican State Committee, serving on several victorious campaigns. In 1896 he was elected chairman of the Massachusetts Republican delegation to the St. Louis Convention, which nominated William McKinley for president.