The fact is there wasn't any Democrat who could even beat President Nixon in 1972, or even give him a tough race, was because of the disarray in the Democratic Party between it's center-Left and Far-Left. Similar to how the Republican Party is today. And there wasn't a Democrat who could bring those two sides together. But even without the emergence of the McGovernites that put all of their support behind Senator George McGovern in 1972, I think they would have a hard time defeating President Nixon. Because of the emerging Southern base in the Republican Party and that the Democrats hadn't locked down the Northeast and West Coast, as well as big Midwestern cities as far as their base. African-Americans and Latinos, were still voting Republican in 1972.
Compared with the late 1960s at least 1972 looked like a fairly peaceful and establishment friendly year. And when that is the case the party in power and that is the party with the presidency, tends to do well. Even if the young Baby Boomers and the broader New-Left in the Democratic Party felt differently. The Vietnam War was ending, America was negotiating with Russia and China and opening up a relationship with the People's Republic of China. The country by in large felt pretty good. The Great Deflation of the 1970s that basically hammered the American economy from really 1973 on, hadn't happen yet. So when the country is like this they tend to feel fairly good and aren't looking for a change in leadership.