How was slavery a states’ rights issue?

It was, not slavery but the issue of states'rights, of which the slavery issue was the main concern. Slavery was deplorable, but, at the time, the states had the right to choose to be free or slave. Only the states assembled in a Constitutional Convention could vote to override the federal law of the land.

Opposition to the nra came to a head in 1935, when the conservative supreme court declared the agency unconstitutional, claiming the nra gave the executive branch regulatory powers that belonged exclusively to congress. this decision began a series of court decisions that overturned key elements of the new deal.