Obama takes the most extreme and evil position imaginable on this issue. How could these endorse such a man using a Christianity as a justification?
Because abortion is decreasing in importance for these people, who are mostly bluer-collar workers (not necessarily laborers but not in higher positions.) They will buy the government-solution propoganda, and when they start to hear the Dems start to quote from the Bible they will buy what they are saying.
They're not real Christians. They select and redefine scripture to fit their fabricated version of god. Brian McLaren is one of the primary proponents of the Emergent Church movement, here is an excerpt that might give you an idea where he's at...
I don’t believe making disciples must equal making adherents to the Christian religion. It may be advisable in many (not all!) circumstances to help people become followers of Jesus and remain within their Buddhist, Hindu or Jewish contexts … rather than resolving the paradox via pronouncements on the eternal destiny of people more convinced by or loyal to other religions than ours, we simply move on … To help Buddhists, Muslims, Christians, and everyone else experience life to the full in the way of Jesus (while learning it better myself), I would gladly become one of them (whoever they are), to whatever degree I can, to embrace them, to join them, to enter into their world without judgment but with saving love as mine has been entered by the Lord (A Generous Orthodoxy, 260, 262, 264).
So you see, being a Christian is no longer a requirement to being a christian in the emergent church mindset.