In 1972, McGovern's nomination was assured by his victory over Hubert Humphrey in California. However, this victory was much more narrow than had been predicted, and relative to that prediction, Humphrey came rather close to defeating McGovern in that race.
So, what if Humphrey had narrowly edged out McGovern in California?
I assume the first thing that happens is New York becomes a real competition, and I can see Humphrey winning that election, because now it isn't as clear who the nominee will be. (If I'm wrong about this assessment, I apologize in advanced.)
So Humphrey in addition to his wins IOTL goes to tha convention with at least California and possibly New York under his belt.
What happens in Miami?