How do you define a Democratic front runner? It’s like Musical Chairs, the last person not to have made a fool of themselves.
The way the nomination system works, it is a competition where the last man standing will benefit.
Historically, there are at most three serious candidates remaining after the Iowa caucuses and New Hampshire primary. The field is narrowed down as time goes on. If you are a survivor then you have a real shot at the nomination just by attrition of your competitors.