The Democrats didn't have anyone who could beat Richard Nixon in 1972. They also didn't have a united party that could do that as well. One of the reasons why Senator George McGovern as leader of the party for that fall campaign reformed the rules for how delegates were assigned at conventions was to bring in more Americans to the party. The party was transitioning from a Southern based party with Northeastern ties to a party that by the 1980s was heavily dependent on the Northeast and West Coast to win. As well as big cities in the Midwest. Because Senator McGovern brought in African-Americans, Latinos, Jews and Asians to the party from these big cities in the country.
George McGovern whatever you think of his politics was a very bright man. And served South Dakota very well in Congress both in the House and later in the Senate. And I'm sure at the very least knew that even if he did win the Democratic nomination for president he had not an uphill battle against President Nixon in the fall. But more like a swimmer trying to swim upstream with one arm, one leg and one eye. But what he did in 1972 as far as changing the voting rules in the party paid off really well for Democrats in 1976 with Jimmy Carter. And perhaps helped Democrats hold onto the House of Representatives for another 22 years after 72 with all of the new voters that came to the party. But 1972 was not a year that Democrats were prepared to win back the presidency. They were in transition and way too divided.