Complex sentence: Denny was elected as a Democrat to the Fifty-sixth Congress (March 4,1899-March 3,1901), but was an unsuccessful candidate for reelection in 1900 to the Fifty-seventh Congress.
Split into simpler sentences: Denny was elected as a Democrat to the Fifty-sixth Congress (March 4,1899-March 3,1901). He was an unsuccessful candidate for reelection in 1900 to the Fifty-seventh Congress.