How would the practice of nullification threaten the union?

The practice of nullification, where states claim the right to invalidate Federal Laws, threatens the union by undermining federal authority and creating a patchwork of laws across states. It can lead to conflicts between state and federal governments, eroding the principle of a cohesive national policy. If states can selectively ignore federal mandates, it risks disunity and could encourage further secessionist sentiments, ultimately destabilizing the union. This challenges the fundamental concept of a united nation governed by a central authority.