California will be key to this nomination. Hillary has the advantage and is "expected" to win. There are 3 scenarios here: 1) If BO wins this either through delegate count or popular vote, this nomination will be his. 2) If Hillary wins the popular + over 5% of delegates the nomination is hers. 3) Hillary wins popular but less than 5% of delegates, this will be knock down, drag out, that will eventually make BO the nominee, as money and name recogntion of BO continues, will begin to favor him. She needs the knock OUT.
Two key factors that I will look for: 1) early votes (greatly favors Hillary) 2) right leaning independents that can't vote (R) will move to BO.
California is the easiest for the pundits to wrap their arms around. They have already stressed the importance here, and will be looking at this as their measuring stick.