From Josh Marshall of TPM
Now, 'contested convention' doesn't have any real technical meaning. But it certainly seems like Sanders is saying there will be no move to building party unity in advance of the convention (put that off to August, apparently) and his campaign will force genuine votes on the floor of the convention while pressing Super Delegates to back him.
Let's set out explicitly what this means: there is simply no way that Sanders can catch up with Sen. Clinton in either delegates or the popular vote. He may prevent her from getting to 50%+ of the pledged delegates, though I'm skeptical of that. But she will come to the convention as the clear winner in delegates and the popular vote. That means - and the Sanders campaign is saying this openly - that their plan is to get Super Delegates to overrule the results of the primary selection process and make him the nominee.
First of all, there's simply no way that is going to happen. This is why the whole super delegate question has been largely bogus from the beginning. You can say all these super delegates support Clinton. But there's simply no way they wouldn't flip to Sanders if he had a clear majority of the delegates, especially if he also had a clear majority of the popular vote. It's not just me saying that. Look what happened back in 2008. Virtually all the Clinton super delegates eventually flipped to Obama.
The case where supers might really come into play would be a situation where one candidate had a thin lead in delegates but had clearly lost the overall popular vote. The popular vote has no formal meaning in the process. But such a mismatch between the two outcomes might allow the super delegates to feel they could get away with or even have an obligation to step in.
http://talkingpointsmemo.com/edblog/what-is-sanders-thinking