https://en.m.wikipedia.org/wiki/Abolitionism_in_the_United_States
Here are a few names of early Americans who criticized or advocated against slavery to one degree or another:
—Thomas Paine
—Benjamin Franklin
—Thomas Jefferson
—William Lloyd Garrison
—Henry Clay
—Elijah Lovejoy
—Frederick Douglass
—Harriet Tubman
—Harriet Beecher Stowe
—John Fremont
—John Brown
—Ralph Waldron Emerson
—Henry David Thoreau
—Abraham Lincoln
—many, many others.
Without doing too deep of a dive: Many of these folks with the exception of those who were themselves black still, unfortunately, held attitudes about blacks that we would recognize today as white supremacist or white separatist.
Some were motivated primarily by religion; others by humanism; others because they lived through it; others a mix.
So, whether you think opposition to slavery makes them “liberal” or not is up to you.
Due to their opposition to a racist hierarchal institution of society, I think it makes sense to conclude they were all liberal for their time, but not by modern standards.
