Those who primarily believe in the social justice part of Christianity to the exclusion of all else are Democrats, while those who oppose abortion and value innocent human life above all else are Republicans (except in the case of capital punishment and war). If ever one of the two parties is able to consolidate those two wings of Christianity ... watch out.
The other issue is spending. We have been having a debate in this country for my entire life on the best way to grow the economy. Liberals have pushed for more government spending facilitated through higher taxation, Republican have argued for smaller government and lower taxation. Both sides believe they are right, and think if only their way is followed the government and the economy will flourish. But the fact is, liberals are wrong, and in being wrong they are their own worst enemy, because if they would only adopt the financial policies conservative Republicans know will work.. they would have far more money to pay for the programs they love so much.
God help the Republican party if the Democrats ever flip to pro-life and adopt conservative financial policies. (I know it's not likely to happen)
But honestly... it wasn't that long ago that I would have thought a black president wasn't a possibility in my lifetime, that sodomy would be accepted as normal, and that pot would be legalized and promoted as medicine.
Change can sometimes happen fairly quickly, and if the poll numbers continue to slip on abortion... don't be shocked if one day the Democrats suddenly flip.
The Christian vote depends on the denomination.
Evangelicals are about a 75-79% republican vote, some denominations vote democrat.