If former President Carter is, as he says he is, our brother in Christ, then that relationship is of greater importance than his political leanings because it is literally established in the blood of Christ Jesus.
To be clear, you can be a Christian and be wrong about a lot of stuff if you believe that you personally are a sinner,saved by grace.
True, it's very possible he is a brother and his sin is covered by the blood of Christ. But I'm not sure that means we ignore the sin.
Most Christians believe sin is an issue, but seem to compartmentalize political sins. That's a bit of a problem because some of the worst atrocities in history have been committed through political means. 60 million murdered unborn babies since RvW for instance. That is doubled the murders of Hitler and Stalin combined. If a Nazi claimed to be a Christian and wanted to speak at LU, should we embrace him also??
So, while it's possible for a Christian to be pro-abortion, abortion is still a sin that needs to be called out. We don't simply dismiss sin just because it's committed by a Christian.