You can find the answer to this in the very last paragraph of this letter.

What Wendell Phillips is saying is that he hopes that New England will secede from the United States.  He wants to do this because he thinks that slavery is so evil that they should not remain in the same country with the slave owners.

Once they have cut themselves out of the US, he wants them to become a place where slaves can come after escaping.  He wants New England to be "an asylum for the oppressed."  He hopes that every slave will dream of escaping to New England.