The Dems' origins were both fundamentally evil---because one way or another they supported slavery---but also because they inherently ENSURED "BIG GOVERNMENT" by the patronage system, wherein government would expand with every election as people gave away jobs to get elected.
The Whigs were no better, as they had no "official" position on slavery. Only the Republicans---for all their flaws---stated unequivocally their opposition to slavery. They may have advocated new "big government" programs, but their MORAL stand was clear and unchanging, just as today, the GOP is the only party to take a clear pro-life stand.
Actually, I think the Republicans' platform resisted the EXPANSION of slavery (primarily into Kansas and Nebraska Territories). They were less strident in their criticism of the existing institution, although certainly more grounded in their opposition than the Whigs had been. Lincoln, for example, never ran on an abolitionist platform, even though his newly formed Republican Party was founded or encouraged by the William Lloyd Garrisons of the era.