[colorado tanker]

Or are you suggesting there would have been a slave-owning South into the 20th century?

Brazil darn near made it into the 20th Century with slavery. No reason the South wouldn't have.

[eno_]

If the new territories admitted to the Union were all free states, the slave states would have found themselves on the economic and political outs pretty quickly.

In the Union or not, slavery was doomed far more quickly than in Brazil. And it is easy to imagine that once the South ended slavery, it would federate with or in some other way reconcile with the North. And with that outcome, an FDR New Deal might never have happened, nor a Great Society... And we might today have something like a government that fits inside of its constitutional limits.

[GOPcapitalist]

Brazil darn near made it into the 20th Century with slavery. No reason the South wouldn't have.

That is a logically unsupportable and speculative assertion. Brazil's emancipation process occurred in the 1880's and was the last country in the western world to have had the institution by law. Even if one supposes that the south had continued slavery to that point, it is an unlikely scenario that they would have been the lone holdout in the western world for much longer against a tide that, between roughly 1820 and 1890 swayed every single other western nation on earth.