With a POD no earlier than June 15, 1962 (the Port Huron statement), your challenge is to avoid the takeover of the Democratic Party by the New Politics/New Left in the late 1960s and early 1970s.
Just because McGovern won the 1972 presidential nomination, that doesn't mean that the New Left "took over" the Democratic Party. McGovern was a traditional liberal who had the support of labor unions and farmers. McGovern had proposed a Demogrant program that would replace the welfare bureacracy and the personal income tax exemption with a $1,000 tax credit for every American. He picked Senator Eagleton and Kennedy inlaw Sargent Shriver as running mates not Tom Hayden.