That bad, eh?
Well, I think its clear that McGovern would have lost, and lost badly, no matter who was his vice-presidential pick. But I think its important to recall that McGovern was considered an interesting figure- perhaps too far to the left- but an intriguing alternative to Nixon. It wasn't until the Eagleton decision that McGovern got tagged with the reputation of being a bungling radical speaking out of both sides of his mouth (e.g. "I support Eagleton 1000%"). Without that narrative, McGovern still loses badly, but not ignobly.
Let me suggest this as McGovern's ceiling, with Muskie on board:
He carries his home state of South Dakota and Muskie's home state of Maine. Muskie's moderation, the respect and name-recognition he earned as the running-mate in 1968, and his Catholicism helps win keep Connecticut, New York, and Michigan customarily Democratic. South Dakota's DFL-dominated neighbor to the east, Minnesota, goes to McGovern, as does Wisconsin, the site of his breakout primary win- between McGovern's grip on Madison liberals, and Muskie's attraction to working-class ethnics, its enough to win over the Badger State.
