Why was the wilmot proviso divisive to congress

Northern states and southern states were at odds over the potential spread of slavery in new territories and what that would do to the balance of power in Congress.  

In 1846, Congressman David Wilmot of Pennsylvania introduced the proviso as an amendment to an appropriations bill in connection with the peace treaty being negotiated with Mexico.  His amendment stipulated that any territory gained from Mexico would be free, not allowing slavery.  Wilmot's amendment passed in the House of Representatives, but was unable to get approval in the Senate.

The Compromise of 1850, a package of five bills passed by Congress in September of that year, sought to accommodate some of the issues both South and North were debating over during those years.  But we know that war over the slavery issue was yet to come.