Complex sentence: He would only serve one term, leaving office in 1853 but during this time he joined the "Opposition Clique", a faction of southern politicians in the Democratic Party who opposed secession and the reopening of the slave trade.
Split into simpler sentences: He would only serve one term, leaving office in 1853. During this time he joined the "Opposition Clique", a faction of southern politicians in the Democratic Party who opposed secession and the reopening of the slave trade.