What was the wilmot proviso? why was it so controversial?

The Wilmot Proviso was a more-or-less religious declaration that slavery wouldnot be permitted in any of the new territories that were likely to be admittedto the USA.
 This would increase Northern dominance of Congress, and enable moreraising of taxes on imports - the big controversy in the South, which had nomanufacturing industry, and depended largely on imported goods.
Please mark brainliest, thanks :)