Bernie Sanders has two possible paths to win the democratic nomination. The first, which has received much discussion, would be to win more Pledged Delegates than Hillary Clinton and hope that the Super-delegates will not impose their will over the will of the people. The other would be to reach more than 50% of total delegates, both pledged and super-delegates.
Total Delegates (per Wikipedia)
To exceed 50% of the total pledged delegates, Sanders would need an additional 988 pledged delegates This would require that on average he wins 57% of the remaining pledged delegates. Although this sounds like a significant challenge I believe that it is achievable. To win the nomination, he would still need the support of an additional 325 super-delegates. He would have to convince uncommitted super-delegates to support him, and persuade super-delegates that currently support Clinton to switch their support to him.
It is reasonable to assume that the majority of the 209 uncommitted super-delegates are likely to support Sanders. If 80% of those super-delegates end up supporting him, he will only need to convince 158, or 1/3, of those currently committed to Clinton to switch to him.
Of the 473 super-delegates that currently support Clinton, 221, or 47%, are elected and are therefore more likely to care how their support might be viewed by the voters. Although some of 252 unelected super delegates might change, many of these are lobbyists, employees of the Clinton campaign and individuals who are otherwise less likely in general to switch to Sanders.
Alternatively, Bernie can get enough pledged delegates and super-delegates to exceed 50% of total delegates. This would require that he gets an additional 1,313 delegates. Again, if he is able to get 80% of the uncommitted super-delegates, he still would need to get 66% of the remaining Pledged delegates. However, if he can also convince 20% of the super delegates which currently support Clinton to switch their support to Sanders he would only need to get 57% of the remaining pledged delegates.
If Sanders does get 57% of the remaining unpledged delegates, he will have an strong case to persuade Superdelegates to throw their support behind him. By winning 57% of the Pledged delegates, he would have won on average one third more delegates than Clinton over the remaining primaries, on top of winning 5 of the previous 6 primaries. In addition, Sanders has a much higher favorability rating than Clinton and the polls have consistently shown that Sanders performs stronger against each of the Republican candidates than Clinton.
Both of these paths ultimately end up at the same point, with Sanders exceeding 50% of the total delegates. They are simply two different ways of envisioning that result.