Substantive due process operates along two distinct branches. The first, the fundamental-rights branch, subjects government action burdening a right deemed deeply rooted in history and tradition, and carefully described, to the most exacting level of judicial scrutiny. The second, sometimes called the arbitrary-government-action branch, asks whether executive or other government conduct -- often in fast-moving or emergency circumstances -- was so egregious and arbitrary that it 'shocks the conscience,' independent of any specific enumerated right.

Courts describe the fundamental-rights branch as one to be extended with great reluctance, requiring careful, narrow description of the asserted right and a demonstrated basis in the nation's history and tradition rather than a broadly stated interest. The applicable level of scrutiny for a claim, once a right or classification is identified, is a related but separate question -- addressed generically in rational basis versus strict scrutiny review.

The fundamental-rights branch protects a narrow, historically grounded set of interests from government infringement absent a sufficiently compelling justification. The arbitrary-government-action branch instead scrutinizes the character of the government's conduct itself -- its egregiousness, its relationship to any legitimate government purpose, and the circumstances under which the decision was made -- rather than asking whether a specific enumerated right was burdened.

Whether conduct shocks the conscience is highly context-dependent, and courts often apply something close to a sliding scale tied to how much time officials had to deliberate. Conduct taken in a split-second, high-pressure setting is measured differently than a decision made with time for unhurried deliberation, with the latter subject to somewhat closer scrutiny. Ordinary negligence, by contrast, essentially never satisfies this standard.

A plaintiff proceeding under the fundamental-rights branch must carefully define the asserted right and ground it in historical or traditional recognition; a plaintiff proceeding under the arbitrary-action branch must show conduct extreme enough to be constitutionally conscience-shocking, not merely wrong or even tortious. Defendants attack by arguing the asserted right is too broadly framed to qualify as fundamental, or that the conduct, however regrettable, falls short of the demanding conscience-shocking threshold.

Because the fundamental-rights and arbitrary-action branches are analytically distinct, with very different proof burdens, a matter alleging a substantive due process violation is best modeled as two separate claim paths rather than one. In Juricratic terms, a user can run the fundamental-rights path (with its own dial for how well the asserted right maps onto historical recognition) and the shocks-the-conscience path (with a dial for the egregiousness of the conduct and the deliberation-time context) side by side, and compare how differently the modeled exposure looks under each theory.