Murray Rothbard, in For a New Liberty, said:

Moreover, principle and strategy merged in the decay of eighteenth-centure and early nineteenth-century liberal devotion to "abolitionism"-to the view that, whether the institution be slavery or any other aspect of statism, it should be abolished as quickly as possible, since the immediate abolition of statism, while unlikely in practice, was to be sought after as the only possible moral position. For to prefer a gradual whittling away to immediate abolition of an evil and coercive institution is to ratify and sanction such evil, and therefore to violate libertarian principles. As the great abolitionist of slavery and libertarian William Lloyd Garrison explained: 'Urge immediate abolition as earnestly as we may, it will, alas! be gradual abolition in the end. We have never said that slavery would be overthrown by a single blow; that it ought to be, we shall always contend.'[4]"
[4]Quoted in William H. Pease and Jane H. Pease, eds., The Antislavery Argument (Indianapolis: Bobbs-Merrill Co., 1965), p. xxxv.

Emphasis Rothbard's.
Powerful argument.