Obama is being razzed by the usual suspects for saying that the theological, scientific and moral question of when human life becomes a human person is "above his paygrade." A classic response:
News flash: There's not a job on the planet above the pay grade of the President of the United States. If you can't solve every problem and are humble about it, that's fine but you can't get away with being unsure about the most defining moral issue in politics.
But even the Vatican doesn't claim to know that precise answer. From the lips of Ratzinger:
"The Magisterium has not expressly committed itself to an affirmation of a philosophical nature [as to the time of ensoulment], but it constantly affirms the moral condemnation of any kind of procured abortion."
So it's above the Pope's pay-grade as well. That "moment" of conception is, of course, many moments in a continuum. Here's how Steven Pinker describes the actual science in "The Blank Slate":
"Just as a microscope reveals that a straight edge is really ragged, research on human reproduction shows that the 'moment of conception' is not a moment at all. Sometimes several sperm penetrate the outer membrane of the egg, and it takes time for the egg to eject the extra chromosomes ... Even when a single sperm enters, its genes remain separate from those of the egg for a day or more, and it takes yet another day or so for the newly merged genome to control the cell. So the 'moment' of conception is in fact a span of twenty-four to forty-eight hours."
The assumption of Obama's critics is that a president should always reduce complex issues to simple black and white truisms, unfounded in reality. That's why they supported Bush. And that's why they're supporting McCain.