His problems aside, McGovern was kinda pointing in the right direction in that the South was a lost cause in the path to an electoral majority- inb4 Carter, the difference between '76 and '72 being widespread distrust in government making a moralizing outsider appealing to the electorate and conservative backlash to Ford, and Clinton's wins were more due to improvements in the suburbs nationwide. That's not to say Muskie or Humphrey wouldn't have a better map, but they would only have improved marginally trying to run a campaign the old New Dealer way. I suspect even without Chappaquiddick that Ted Kennedy wouldn't be able to recapture his brother's quintessentially early '60s magic. It's too early to pivot to the right, so Jackson loses both the New Left activists and organized labor while failing to improve in the South.
Conventional wisdom seems to be Muskie and that's probably accurate. Absent Nixon's scandals catching up with him earlier, which fundamentally changes 1972, a really good Muskie campaign that goes on the offense and distracts Northern Democrats from their divisions exhibited during the Hard Hat Riot probably maxes out here, give or take Pennsylvania:
President Richard Nixon (R-CA) / Vice President Spiro Agnew (R-MD) ✓
Senator Edmund Muskie (D-ME) / Senator Walter Mondale (D-MN)