While Jimmy Carter was remembered as being a staunch leftie liberal progressive, he really wasn't. Bernie Sanders, he wasn't.
While governor of Georgia he was a huge & vocal supporter of the Vietnam War. Most governors were fairly quiet about the national issue. Only California's Ronald Reagan and Texas' John Connally (whom, like Carter, had eyes on the White House) were as strong in their hawkish support of the war.
Governor Carter was a supporter of William "Rusty" Calley in 1971 when the butcher of My Lai was indicted for war crimes by the US Army, to his eternal shame.
In 1972 Carter & a lot of other Democrats (Ted Kennedy, most prominantly) figured, rightly that Dick Nixon was a lock for re-election. By 1976 Carter could read the pulse of the nation, i.e forget about Vietnam altogether & campaign as an uber-lib (though he still wasn't & didn't govern as president as one).