Lincoln believed that President Polk had acted unconstitutionally when he declared war against Mexico over the annexation of Texas. President Polk did not go through the standard process of requesting an act of war through Congress, but instead used his executive powers to initiate war. In addition, Lincoln did not want to introduce another slave state into the Union.
Southern Democrats posed the argument of Manifest Destiny as a challenge to that stance. According to Manifest Destiny, the U.S. has a moral obligation to annex as much territory as possible in order to spread the ideals of liberal capitalism and democracy. Since Texas had been a disputed territory for several decades, many southern Democrats also argued that Mexico had no right to the land of Texas and that the territory rightfully belonged to the United States.