Complex sentence: He was not a candidate for renomination in 1924, but was an unsuccessful candidate for the Republican nomination for United States Senator; subsequently elected to the Sixty-ninth Congress to fill the vacancy caused by the death of Harry Irving Thayer.
Split into simpler sentences: He was not a candidate for renomination in 1924, but was an unsuccessful candidate for the Republican nomination for United States Senator. Subsequently he was elected to the Sixty-ninth Congress to fill the vacancy caused by the death of his successor, Harry Irving Thayer.