What? Are you actually calling Carter and Clinton centrists? LOL!!!
Compared to Barack Obama and Bernie Sanders, probably. Bill Clinton was an opportunist, he was not an ideologue. His wife? A different story.
Jimmy Carter was just an idiot but the point is that in 1968 and 1972, the Democratic Party drifted to the Far Left. So Jimmy Carter appeared to be in the center when he ran for President in 1976 even though many of his policies during his administration weren't.