NG Style Manual‎ > ‎- A -‎ > ‎

Amendment (U.S. Constitution)

Spell out through Tenth Amendment. An enacted and ratified amendment is capitalized when its number is used:
            the Fifth Amendment
            the 18th Amendment

Lowercase informal titles unless they include words that are capitalized in their own right:
            income tax amendment
            the Prohibition amendment

A proposed constitutional amendment should be lowercase:
            equal rights amendment (ERA)
            the Bricker amendment