I wrote about exit polls in another post a while back–I’l see if I can dig it up. In the meantime, a summary:
Basically, exit polls are not in the least predictive, and are not meant to be. Despite assertions from Sanders supporters and statements from people like Robert Kennedy Jr., there is no reason to think that exit polls ought to accurately reflect the actual vote count. This is the case for several reasons, including the fact that very few polling places in any given state are actually surveyed.
In this particular case, there are a number of reasons to believe that exit polls significantly overstated Sanders’s actual level of support, among them:
–Younger people are known to be much more likely to respond to requests from exit pollsters than older people, and left-wingers are more likely to respond than more conservative people. The typical Sanders voter (very leftish and younger) is thus more likely to be included in an exit poll than the typical Clinton voter.
–It is really difficult to follow protocol at a crowded polling place (ask every tenth person…), leading to potential unconscious bias on the part of the people hired to carry out the polls–who may be more inclined to seek out people who are most like them. I don’t have a breakdown, but I have seen indications that many of the interviewers are young and white–the profile of the typical Sanders voter.
–In states where early voting is possible, indicators are that Clinton did especially well among people who took advantage of this opportunity. People who voted on the day of were disproportionately more likely to be voting for Sanders. Again, exit polls would inflate the actual share of Sanders’s votes as it would not include the people who voted earlier in the polling period.
–Exit polls do change as the day goes on as different waves of voters show up at the polls. There is some evidence that Clinton voters may have been more likely to vote later in the day, which would make an early Sanders edge in exit polling seem to disappear as the day goes on–which might look suspicious to someone primed to look for irregularities.
The other important thing to keep in mind is that exit polls do not truly represent independent events. The 1 in 77 million or whatever figure is based on the notion that the exit polls indicated fraud in New York, and in a completely separate event that the Illinois exit polls indicated fraud there, and so on and forth, and so we can multiply the odds in each state and arrive at some frankly overwhelming figure with an enormous number in the denominator. Of course, you can’t, because the biases everywhere all point in the same direction: the direction of exaggerating Sanders’s support. If we walk down the street and see 17 Chinese restaurants and no others, perhaps it’s a conspiracy of some kind, or perhaps we’re simply in Chinatown–or in China.
That’s a part of the answer. I’ll see if I can find that other post and link to it. Suffice it to say for now that there are very good reasons in general not to trust the exit polls, and very good reasons in particular not to accept that they demonstrate any kind of fraud designed to screw Sanders out of the nomination. Hope this helps.