There is one argument that is primarily directed at Northerners, and that is that
Deuteronomy 23:15-16 prohibits the return of fugitive slaves.
According to every interpretation I've seen, modern or antebellum, that isn't the true meaning.
http://www.studylight.org/commentaries/mhm/view.cgi?book=de&chapter=023
I. The land of Israel is here made a sanctuary, or city of refuge, for servants that were wronged and abused by their masters, and fled thither for shelter from the neighbouring countries,
Deuteronomy 23:15,
16. We cannot suppose that they were hereby obliged to give entertainment to all the unprincipled men that ran from service Israel needed not (as Rome at first did) to be thus peopled. But, 1. They must not deliver up the trembling servant to his enraged master, till upon trial it appeared that the servant has wronged his master and was justly liable to punishment. Note, It is an honourable thing to shelter and protect the weak, provided they be not wicked. God allows his people to patronise the oppressed. The angel bid Hagar return to her mistress, and Paul sent Onesimus back to his master Philemon, because they had neither of them any cause to go away, nor was either of them exposed to any danger in returning. But the servant here is supposed to escape, that is, to run for his life, to the people of Israel, of whom he had heard (as Benhadad of the kings of Israel,
1 Kings 20:31) that they were a merciful people, to save himself from the fury of a tyrant and in that case to deliver him up is to throw a lamb into the mouth of a lion. 2. If it appeared that the servant was abused, they must not only protect him, but, supposing him willing to embrace their religion, they must give him all the encouragement that might be to settle among them. Care is taken both that he should not be imposed up on in the place of his settlement--let it be
that which he shall choose and
where it liketh him best, and that he should not exchange one hard master for many--
thou shalt not oppress him. Thus would he soon find a comfortable difference between the land of Israel and other lands, and would choose it to be his rest for ever. Note, Proselytes and converts to the truth should be treated with particular tenderness, that they may have no temptation to return.
http://www.freebiblecommentary.org/old_testament_studies/VOL03OT/VOL03OT_23.html
The central interpretive question is the nationality of a slave and of his master. To whom does this exactly refer? This must refer to a foreign slave or a foreign slave-master (or both). This does clearly show Israel's understanding that a slave is more than an animated tool. YHWH allows servitude under certain restrictions and limits, but He also cares for the powerless, helpless, and vulnerable!
http://biblehub.com/commentaries/deuteronomy/23-15.htm
This is not to be understood universally, as if all servants that flee from their masters, though without any sufficient cause or colour of justice, might be detained from them by any person to whom they fled for refuge, for this is apparently contrary to all the laws of religion, and justice, and charity, and would open a door to infinite disorders and mischiefs