Following the previous election in 1859, Republicans had control of the Iowa Senate with 22 seats to Democrats' 21 seats. However, four changes occurred during the eighth general assembly. In the ninth district, Republican Senator Alvin Saunders resigned on May 14, 1861, causing a vacancy in his seat. Republican Leroy G. Palmer succeeded Senator Saunders, holding the seat for the Republicans. In the tenth district, Republican Senator James Falconer Wilson resigned on October 7, 1861, causing a vacancy in his seat. The tenth district seat was left vacant until a special election in 1861. In the fourteenth district, Democratic Senator Andrew Oliphant Patterson resigned on May 14, 1861, causing a vacancy in his seat. Republican Joseph A. Green succeeded Senator Patterson, flipping the seat to Republican control. In the thirty-fifth district, Republican Senator Thomas Drummond resigned on May 14, 1861, causing a vacancy in his seat. Republican Joseph Dysart succeeded Senator Drummond, holding the seat for the Republicans. Therefore, by election day in 1861, the Republicans held 22 seats, the Democrats held 20 seats, and one seat was vacant (the seat that had been held by Republican Senator James F. Wilson).
