Americans who opposed the Constitution were called:

Whigs

Antifederalists

Federalists

Americans who opposed the Constitution were called Antifederalists. They were a select group, a coalition, so to speak, of people who opposed ratification of the Constitution. It's true that they were less organized that Federalists, but they still had an impressive group of leaders who were especially prominent in state politics.

They are called Antifederalists. The Antifederalists were a differing coalition of individuals who restricted endorsement of the Constitution. Albeit less efficient than the Federalists, they likewise had an amazing gathering of pioneers who were particularly unmistakable in state legislative issues.