Since TheNation.com seems intent of denying it.

... and since one cannot read any of their articles without paying for it ...

http://www.truthdig.com/report/item/why_cant_the_nation_the_left_deal_with_election_theft_20160418

Hmm ....

We plead guilty to writing a piece at Reader Supported News pointing out that the exit polls show Bernie doing far better than the official vote count. Josh dismisses the reliability of exit polls, along with our mention of those in Germany. Suffice it to say that exit polling is the most reliable way we have of monitoring the vote count. In Germany they are usually within 1% of the official tally.

Here they are corrupted after the original compilations. They’re generally post-rigged to conform to the final official vote count, which is often pre-rigged. What matters is the original raw data, which we feel after much study is the gold standard for knowing how the public actually voted.

Of course that data can be messed with along the way. But at this point it’s the best we’ve got. And the original data can be hard to get. When statistician Ron Baiman requested original data from big-time pollster Warren Mitofsky (since deceased) Mitofsky told to him to “go fuck yourself.”

Of course it's possible there's electoral fraud. Both the Democratic and Republican Parties in the US have been around for over 150 years. They are established organizations with very strong political machinery, both of which have been severely co-opted and corrupted by the larger corporate establishments. 

So of course it's no surprise that there's a chance there's been electoral fraud in the primaries (not just New York, but Arizona, Colorado), when outsiders who were never really part of them, nor such people ever really kow-tow or submit to their will, suddenly turn around and challenge their supremacy over voters.