Complex sentence: He was an unsuccessful candidate for reelection in 1854 to the Thirty-fourth Congress, but was elected as a Unionist to the Thirty-seventh Congress (March 4,1861-March 3,1863).
Split into simpler sentences: He was an unsuccessful candidate for reelection in 1854 to the Thirty-fourth Congress. May was elected as a Unionist to the Thirty-seventh Congress (March 4,1861-March 3,1863).