She now has 1,645 pledged delegates. You need 2,383 delegates to clinch. So, if she doesn't use super delegates, then she needs 738 of 1,206 remaining delegates up for grabs. She is very unlikely to win that many delegates in the upcoming states, which largely favor Bernie Sanders.

That means the first thing Sanders supporters have to do is make sure he wins those states and picks up as many delegates as he possibly can to block her winning the outright majority with just pledged delegates.

At this point, you have to be wondering, "Yes, but what about the super delegates?! They're almost all on her side." Yes, of course, that's why Sanders' chances of winning are not very high. But the whole point of those super delegates is to make sure that the Democratic Party does not have a deeply flawed or hobbled candidate going into the general election. Their job is not to overrule the will of the voters because they don't happen to agree—that's why a lot of them switched from Hillary Clinton to Barack Obama in 2008 when he won a majority of the pledged delegates.