Christians in this country have been voting mostly for republicans for as long as I can remember, since the 1980's at least. Why? Republicans are perceived to be supporting "biblical" principles, namely: Stop abortion, prevent gay marriage, be fiscally responsible (i.e. get rid of the national debt), and so on. Many people I know vote or have voted republican on the first one alone. (And obviously I know that there are arguments pro and con for these social issues, from both Christian and non-christian stances; whether these are good things or not is irrelevant to the point I'm making here.)
Since 1980 we've had 20 years of Republicans in the white house and only 8 years of Democrats. And the Republicans have not done any of the things those Christians wanted them to. Abortion is still legal, the national debt is bigger than ever, etc.
With the current Republican administration being one of the worst administrations this country has ever had, Christians are going to do some math. What does 20 years of inaction = ? It = Republicans aren't going to do anything on the issue and probably never will, either (for good reason; it would remove one of their main issues, not to mention pissing off everyone else that voted for them). Voting for the democrats is no more a vote to keep killing babies than any other vote would be, based on the last 28 years, anyway.
It is objectively clear that the republicans, as a group, have spurned the will of one of their largest blocks of support. Why would that block keep voting them in? I don't think they will.
With that said, I'm very happy that Obama is going to get the dem nomination and not Clinton. We've already had a round 2 of a recent president. Let's not do that again.