Souter was a moderate when GHW Bush nominated him but turned into a liberal along the way. Happily for us, GHWB also nominated Thomas who cancels Souter out. Clinton got in two liberals which bumped the court leftward. Reagan put in one conservative, Scalia and two swing voters, Kennedy and O’Connor.
Unhappily for us, older conservatives can degenerate into swing voters, but older liberals never improve.
Souter was never a moderate. Howard Phillips exposed him at the time of the nomination, as I recall. He was “moderate” only in the sleepy eyes of John Sununu (Sr.)