Carter believes that if Dems took this position that the party would win back conservatives it has lost over the abortion issue.
Carter is attempting to appeal to a set of voters that doesn't exist. There just isn't a significant number of people out there saying to themselves "I agree with the Democrats on everything except abortion, so that's why I never voted for them." Anyone who bases his/her vote solely on whether a candidate is anti-choice or not will still vote Republican regardless of what the Democrats do. On the other hand, there are a large number of voters in the Democratic camp who would be outraged if the party adopted the fundamentally incoherent position on abortion that Carter espouses.
Bottom line: if I want to build a house or grow peanuts or run an election in a third-world country, I'd turn to Carter for advice. If I want to win an election in the US, on the other hand, he's definitely not the first guy I'd consult.