"Well if that person, human life is not a person, then, I find it almost remarkable for a black man to say, 'We are going to decide who are people and who are not people.'"
Conservatives really like this comparison because it seems like an ironclad way to hoist liberals with their own petard, and I see various ones have already given this comparison their endorsement.
Personally, I still think arguments made by analogy are almost always specious. People usually move from "X is" to "X is like" because they only want you to see this one detail here, not all those inconvenient ones over there.
For example, Coates points out the problems with the "abortion is like slavery" thesis by pointing out that slaves were considered people, just unequal people, and moreover that they were themselves one of the primary agents in their own abolition. It totally glosses over the long struggle of black people escaping slavery and becoming abolitionists, pamphleteering, preaching, and helping other slaves escape bondage via the underground railroad. Thus the "pro-life as abolitionist" model is that of altruistic white men condescending to lift up and extend their own privilege to some helpless, voiceless other, which is understandably offensive to others, not to mention "bad history" as Coates argues.
In this way, Rick Santorum interestingly steps into the same puddle as Hillary Clinton back in '08 when we were all forced to litigate the degree to which Lyndon Johnson (and by extension white America) can or should claim responsibility for the Civil Rights Act. African Americans are very sensitive to this subject because white people tend to understate the role of black people in their own emancipation.
That is all, of course, aside from the more obvious critique that if you decide personhood begins at conception, you're still "deciding who are people and who are not people," and it's not just a matter of setting the start date earlier. If "personhood" includes, say, "the right to make your own medical decisions," there is the wee matter of the mass of flesh surrounding the fetus.
There's yet another wrinkle in Santorum's logic that I thought worth mentioning. Despite the reductionist description of the president as "a black man," Barack Obama is not the descendant of American slaves. There isn't anything in his heritage that would make this issue more acute for him than for Santorum.