What slogan does he proclaim as the motto of any serious abolitionist?

The slogan that is often proclaimed as the motto of any serious abolitionist is "No Union with Slaveholders." This phrase encapsulates the belief that any association or compromise with those who support slavery is unacceptable and that true freedom and justice can only be achieved by entirely rejecting and opposing the institution of slavery. It emphasizes the moral imperative to stand firmly against slavery in all its forms.