Complex sentence: He was elected as a Democrat to the United States Senate to fill the vacancy caused by the resignation of Isaac D. Barnard and served from December 13,1831 to March 3,1833, when he declined to be a candidate for reelection.
Split into simpler sentences: He was elected as a Democrat to the United States Senate to fill the vacancy caused by the resignation of Isaac D. Barnard. Dallas served less than 15 months--from December 13,1831, to March 4,1833--and declined to be a candidate for reelection.