Carter faced a tough fight from The Swimmer for the nomination, I think if Carter ran opposed for the nomination, he might very well have won.
That’s the one good thing The Swimmer did, was to help take Carter down.
Arguably, Kennedy’s challenge diminished Carter’s stature and his hold on the affections of the Democratic Party. Yet those effects ought not to be given too much weight as Kennedy’s challenge does not seem to have shifted any elements of the electorate from Carter to Reagan.