California is an open primary state. s
True.
Bern has upset Clinton too many times to count.
Not if you can count... Clinton has won 23 states, and Sanders has won 18.
Unfortunately, for Mr Sanders, he has lost the six largest state (by population) to hold their primaries so far: Texas, Florida, Pennsylvania, NY, Ohio and Illinois. That goes a long way towards explaining why he is so far behind in the delegate count.
Momentum is on Bern's side.
Possibly - but inertia is certainly on Clinton's side. Even if Sanders wins every remaining state, he can't overtake her delegates unless he wins by double-digit margins in the largest states.
Quite a few polls show single digit leads for Clinton.
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That's very bad news for Sanders, and good news for Clinton. She doesn't need to win to clinch the nomination - she only needs to not loose by large margins in big states.
Polls have been inaccurate up to the very day of primary
QUite possibly, but which polls are you referring to? Which states? How far off? From what I've seen, of the states that have had large polling coverage the polls have generally been within the statistical margin of error. Given the complexity of the primary system many of the small states were not polled very much - including the upcoming Kentucky and Oregon states.
note: I'm not advocating for Clinton here; I just believe in an objective reporting of the challenges each faces.