Sorry to answer your question with a question, but by mainstream do you mean people who opposed slavery in general, abolitionists included, or do you mean just the abolitionists?

The distinction can get fuzzy, but usually the line is drawn between people who advocate the immediate end of slavery (few in number and generally considered extremists at the time) are called abolitionists and those who preferred more gradual means are called antislavery. We inherit the terms from the nineteenth century, which the distinction mattered greatly to the people involved and not at all to their opponents. For example, William Lloyd Garrison is an abolitionist. Abraham Lincoln, and just about every other politician who opposed slavery before the Civil War, is antislavery.