There's an NBC/Wall Street Journal poll out. It shows a 2 point race. The NBC News poll doesn't say what percentage of the Democratic primary electorate is Democrats and what percent is NPP voters. I suspect they are overestimating the NPP voters. Sanders does very well with unaffiliated voters but Clinton tends to dominate with Democrats. Only 17% of the voters so far are NPP/other, down from 20% two years ago. Only 14% of NPP vote by mail voters requested a Democratic ballot. If only 14% of that 17% turn in ballots with Democratic Presidential votes, the vote by mail electorate will be 95% Democratic/5% NPP for the Presidential primary.
It should be better for election day voting because NPP voters will be able to request a ballot in person. I expect election day to be closer to 70-75% Democratic/25-30% NPP. That might not help a lot, because in 2014 VBM was 69% of all ballots. The percentage of VBM voters has been increasing with every election and VBM returns are brisk. The best case for sanders is that the overall ballots are 88% Democratic/12% NPP. I expect it'll be 90% Democratic/10% NPP.
That split should cause Sanders to lose by at least 10 points, but probably at least 15. The Sanders' supporters will claim Clinton cheated because the results won't match the polls. Add to that, Sanders supporters lost their lawsuit today to change registration rules. Bernie will have to live with this electorate. And that should be a disaster for Sanders.