Use the case a constitution's parts take

Warning²Federal courts, Illinois courts21st and 22nd editions

A noun naming a part of a constitution carries a capital where a sentence refers to it — “the Fifth Amendment”, “Article I, Section 8” — and takes T16’s lowercase abbreviation inside a citation.¹

1. 1R. 8(d)(ii) Exceptions; R. 8(c)(ii) Exceptions. Tables: R. 11 Constitutions; T16 Subdivisions.
2. 2A warning: the form is usually wrong under the rule in note 1, and context can justify it.