Perhaps the worst was switching VP candidates -- Thomas Eagleton was apparently dumped for having gone to a psychiatrist ... nowadays we might wonder about a candidate who has not gotten treatment. Anyway, the switch undermined the public estimate of McGovern's ability as a judge of appointees, his loyalty, his decision-making (especially because he dawdled before making the wrong decision), and so forth.
Probably the second worst blunder was the scheduling of his acceptance speech at the Democratic Convention. Somehow dozens of (liberal) pressure groups - Latino, African-American, anti-war, feminist, etc. etc. - persuaded McGovern and his people to give each of them a shot at national TV - in prime time, no less - (as if they would cross over to Nixon if they didn't get their way!!) so they were ALL alloted what was supposed to be no more than two minutes speaking time before McGovern's speech. At the very best this pushed his speech up to around 10 pm Eastern time. But, of course, none of these special interest spokemen could keep himself/herself down to two minutes, so McGovern made his acceptance speech ("Come Home America") past midnight to a half empty convention hall and a lot of cold TV sets. The most crucial speech of his campaign was a total FUBAR.
LOL! His pandering created its own punishment.