Respectfully, I disagree. My analysis is directly related to the 1968 US presidential Election and NOT the 1964 election. LBJ knew he had '64 in the bag. His polls told him. The RINO's told him.
LBJ was worried about what had happened to Harry Truman would happen to him and he would not get a second term, just as Truman was denied one by Korea. If Saigon was lost before '68, Johnson feared, and rightly so, that he would loose to a Republican, "Ike" type. Remember, it is almost the same amount of time from Korea to Vietnam, that it is from the invasion of Kuwait till now.
Lyndon Johnson had Korea, the 1952 election, Saigon and the 1968 election, very much on his mind when he sent US troops into Vietnam in 1965.
Ironically LBJ was handed a potentially decisive victory in 68, Tet, which he allowed the media to turn to defeat. He inherited the war, he clearly didn't want to be the one to "lose" it, but winning, not on the agenda.