Complex sentence: Riggs was elected as an Anti-Lecompton Democrat to the Thirty-sixth Congress, serving in office from March 4,1859-March 3,1861, but was not a candidate for renomination in 1860.
Split into simpler sentences: Riggs was elected as an Anti-Lecompton Democrat to the Thirty-sixth Congress (March 4,1859-March 3,1861). He was not a candidate for renomination in 1860.