It depends on whether it's a proper noun.
Patrick C. asked,"When discussing a Constitutional Amendment, is it instead a 'constitutional amendment'?"
"Constitutional" is lowercase because it is an adjective, but sometimes "constitution" should be capitalized.
When you're using "constitution" descriptively, it's also lowercase:
The chess club needed a new constitution.
We should look that up in our constitution.
In the U.S., when you're referring to the specific founding document we refer to as the Constitution, it is capitalized:
George Washington's name is the first signature on the Constitution.
We can't wait to see the original Constitution when we visit the National Archives.
To directly answer Patrick's question, he should write that something is a "constitutional amendment"—lowercase.
Get more tips like this in The Grammar Devotional: