Video here: https://www.youtube.com/watch?v=7hqYGHZCJwk
Summary: Herschenson argues that up until 1972 the United States was winning the Vietnam war by any calculation. The 1973 Paris Peace Accords, in which the US pledged military aid for South Vietnam, presented the possibility of a US withdrawal that could preserve South Vietnam's security. However, in the 1974 elections, lots of anti-war Democrats were elected, who defunded the aid to South Vietnam. Without the aid, which Ford begged for, the South predictably fell to the North in 1975.
Let me just say that this explanation sounds like a biased oversimplification. Even if the ARVN got the $700 million in arms Ford was proposing, was the South even stable enough at this period to mount a protracted defense? What do you guys think?
Regarding the first point, it's absolute rubbish. Please see my extended post on the counterinsurgency strategy of the US during the Vietnam War. Regarding the second point, it is oversimplified but true to a certain extent. I've written more about it here.