#1) How are they similar to "the slaves of political parties"?
Answer: Slaves wanted to be trusted by their overseers because this often would grant them privileges that other slaves did not have.
#2) Why does Douglass make this comparison?
Answer: Slaves would seek with great care to please their overseers to gain their trust, just as office-seekers in the political parties seek to please and deceive the people.
I hope it helps, Regards.
This is a persuasion. Where he says "To attract new businesses to our shores, we need the fastest, most reliable ways to move people, goods, and information—from high-speed rail to high-speed Internet." he is stating the problem. Also where he says "I'm proposing that we redouble those efforts." he is prompting a solution to the problem. Overall he is stating the problem , giving the process and asking for you to help with the solution.
For ye shall hear, how our heavenly king
Calleth Everyman to a (general reckoning).