To what does he refer when he says (in the last sentence) “the denial of the right of the state to interfere constitutionally in the last resort”?

In the final sentence, Calhoun meant that the denial of the State right to nullify the federal tariff meant there were only two options for the Southerners: they could to submit to northen rule, which meant the end of its cotton economy, or they could rebel against it. It alarmed Coulhon because the two options had devastating consequences.

a historian researching the immigration policies of the early 1900s would expect a source written today,rather than in 1910 to :

( a ) be more interested in objective data and statistics.