What were the political arguments made by the Free Soil Party?
The Free Soil Party was founded on the conviction that slavery should not be allowed to spread into new territories. Free Soilers rather supported wage labor, and its platform was based on the assumption that the existence of slave labor in the territories would undercut white wages in particular and economic opportunities in general.
Though the party did include anti-slavery titans like Frederick Douglass, most Free Soilers were not abolitionists, and even those with moral qualms about the institution of slavery did not believe it could or should be abolished by the federal in the states where it was already established. Additionally, many Free Soilers had less than enlightened views about race by modern standards, and in fact many were as opposed to free blacks entering the territories as they were slaves.
But they also deeply resented the inordinate power that they believed a southern "slave power" exercised over national politics. This conviction was expressed by including "free speech" in their 1848 party motto: "Free Soil, Free Speech, Free Labor, Free Men." Many Free Soilers would find a home in the newly formed Republican Party in the late 1850s, and their views on the expansion of slavery were adopted almost wholesale by the GOP.