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

The Wilmot Proviso, one of the major events leading to the American Civil War,  would have banned slavery in any territory to be acquired from Mexico in the Mexican War or in the future, including the area later known as the Mexican Cession, but which some proponents construed to also include the disputed lands in south Texas and New Mexico east of the Rio Grande. This was controversial between the Southern and Northern states because this allowed the Union to always have more representatives in the House of Representatives.  It would also make sure that the Union could control the Southern states and vote against anything pro-slavery.