by the acts of their Legislatures nullifying the Fugitive Slave Law, and in direct disregard of their constitutional obligations
In his argument using the example of the Fugitive Slave law nullification, Stephans might be referring to the Constitution which is the supreme law of the land in the United States. It creates a federal system of government in which power is shared between the federal government and the state governments. However, Federal law trumps state law when there is conflict.