I think there is a religious justification for being pro-choice, but not pro-abortion. Thus, I can rationalize being pro-choice, but I cannot come up with a Christian rationalization for public funding of abortion.
Clearly, abortion is sin. But God gave humans free will in order that they may distinguish between sins and non-sins. The nation's laws may or may not comport with God's laws. It is not necessary, or even always desirable, that they do. So long as I have free will, knowing God's laws I can take care of my own soul, and use my powers of persuasion to convince others of the error in their thinking. In fact, it would be preferable if abortion were so taboo in our society that we did not have to pass laws to prevent it. Like cannibalism.
I cannot similarly reconcile public funding of abortion. Forcing people of conscience to pay for an abomination is completely antithetical to Christian beliefs, as far as I know them.