Did the Vietnam War cost LBJ his chance of a second full-term or was it his cardiac woes?

It was both, I think. The Vietnam war split the democrat party into four main factions. Right after the Tet offensive started, Walter Cronkite gave an editorial report that the war was now unwinnable. When Cronkite said that Johnson said "If I've lost Cronkite, I've lost Middle America." And he was right. The Democrats lost the white house that year to Richard Nixon. Johnson died of a third heart attack due to heavy smoking, heavy drinking, and lots and lots of stress.