Who the government considered to be an “undesirable immigrant”?

Thus nativism has become a general term for opposition to immigration based on fears that immigrants will "distort or spoil" existing cultural values. In situations where immigrants greatly outnumber the original inhabitants, nativist movements seek to prevent cultural change.

oil! is what i am assuming

the  wilmot proviso  was designed to eliminate  slavery  within the  landacquired as a result of the mexican war (1846-48). fearing the addition of a pro-slave territory, pennsylvania congressman david  wilmot proposedhis amendment to the bill.