There's already a thread devoted to his enneagram but I'll revive this one as I'd like to argue against INFJ as well.
Carter was known for being a policy wonk who was obsessively devoted to details more than he was noted as being a big vision President. Compare him to Clinton (ENFP?), who liked to use metaphorical language like "build a bridge to the 21st century." If you watch interviews, he's very good at remembering details of events that happened decades ago, but not so much focused on the aura, mood, implication of events that happened. Take for instance his supposed UFO sighting in 1969. If you listen to any interview where it's mentioned, his focus is still on the details and what he saw far more than it is on what it meant, what it could've meant, etc. He just knows he say an unidentified flying object, but he isn't as concerned with all of could-be's of what he saw. All of this suggests a preference for sensation.
He was also extremely process-oriented, almost more focused on the means than he was the ends. If it functioned well, that was just as important as what the function of any given process or bureau yielded, and some suggest it was even more important to him than end results. I believe this suggests someone with a Judging temperament over a Perceiving temperament. So now we've narrowed him to xSxJ.
Huge humanitarian, very concerned with the suffering and well-being of others. It visibly moves him, almost as if he feels a bit of others' suffering. Sometimes he can come across as a bit fake in this area, but that could be due to years of being a politician. It could also be due to Fe sometimes appearing less genuine than Fi, even if it isn't necessarily less genuine. So we're at xSFJ now.
Somewhat reserved and slightly less exuberant than I would expect to see in an ESFJ, so I think ISFJ is a good match. A decent President elected at the wrong time, when people really wanted an ESxP cowboy or an ENFx visionary type to boost national morale.
Now, INFJs, sure they are perfectly capable of remembering details. Carter was certainly a good diplomat, and that too could suggest NF, but that doesn't exclude other types from being good diplomats. SFJs can have their own unique flair for diplomacy. And I still wouldn't rate his diplomacy as his greatest talent. He was an administrative wonk with a heart, by most accounts of people who worked with him. I have a hard time seeing an INFJ being quite as obsessed with the minutia of every little piece of policy. I think they'd be more likely to have a broader vision and delegate the details of implementation and process to their staff. Jefferson, who was likely either an INTP or an INFJ, is a good contrast to Carter's style. He seemed to get overwhelmed and annoyed by being involved in too many minor details of policy implementation. He'd rather focus on coming up with a grand vision like Manifest Destiny or a big idea like a transcontinental expedition whilst leaving the particulars of implementation to others. Carter did occasionally "preach," for instance the infamous "Confidence of Crisis" speech where he proceeded to lecture and chastise Americans, but I don't see this as being uniquely NF as much as I see it as an unfortunate exhibition of Fe in full assault. It's not compelling evidence for an NFJ, in my opinion, though it is something an NFJ could be capable of doing in the right circumstances. It exudes Fe:
I really have a difficult time seeing NFP or NFJ for Carter. This could be a classic SFJ-NF mixup, right, [MENTION=10082]Starry[/MENTION] ?