Take this for what you will for someone who was 6 at the time living in Solidly Democratic Appalachia- Barely aware of the election, but it made an impact. McGovern was really hurt by those who seemed to be around him- the counter culture of the 60s. That particular subset really bugged/scared a lot of old New Deal Democrats.
How that came to be can be discussed- I think Nixon's tactics/tricks certainly helped, but McGovern did not help himself at all (The Salinger Mission, amensty for draft dodgers, the snubbing of Richard Daley at the Democratic Convention, the guaranteed annual income proposal, his relations with the Democratic party establishment of the time).
McGovern just wasn't the one to run against Nixon in 72. If by some miracle/ASB intervention he had managed to eke out a win- the 1974 off-year elections would have been a Republican rout followed by Ronald Reagan in 1967 easily defeating McGovern.
I know this is an old topic sorry for rising it if it offends.
Why is this? The only thing I can see is media Hysteria. There is also strong evidence that suggests manipulation in many aspects on the part of NIxon. From plants at rallies to watergate. From the studying I've done it seems to me this was somewhat of an unfair and lopsided election from the get go.
There is a lot the public didn't know about McGovern and a lot of lies that were widely believed. I find the election fascinating.