originally, filley was a "hard money jackson democrat" and a personal friend of thomas h. benton, the democratic u.s. senator from missouri from 1821 to 1851 who was a champion of westward expansion in the united states, a cause that became known as manifest destiny. during the time late 1840s when the wilmot proviso proposed to ban slavery in territory acquired from mexico in the mexican war. filley declared himself in full support of free soil and the emancipation of missouri, leading him to support martin van buren in the presidential election of 1848. the free soil party was eventually absorbed into the republican party.