McGovern was an open sacrificial candidate. No matter what he did, he would lose handily. The only two men with a remote shot at beating Nixon would've been Ed Muskie or Hubert Humphrey.
The Interesting fact of that is before the primaries McGovern reckoned the hardest bit of his task would be getting the nomination.
One of the reasons he did so badly was that the Democratic establishment feared a McGovern presidency as much as they did a Nixon one. Which may explain the Union vote (Despite the myth McGovern did rather well among blue collar workers in the primaries but after the nomination many of the unions sat on their hands - similiar to what Irish and German community Democratic machines did to Cox in 1920.)
McGovern tended needed the "Anti-politican" vote to do well. But after the Eagleton affair that was finished, he came across too much as a phoney. Though he did have 41% iirc in the polls before the affair - which is higher than he actually got. Most potential McGovern voters then stayed at home (very low turnout in 72') leaving him only with hardcore Democrats and moderate-liberal Anti-war types.
I'd Reckon:
(no clue about percentages)
Of course things might have finished very different had McGovern got the person he wanted to be his running mate - Sen. Ted Kennedy (D-MA).