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 racist or even white supremacist.
Some were called to abolitionism by religion; others by humanism; others because they lived through slavery themselves; others a mix.
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 necessarily by modern standards.
