Look, This is Really Simple does a great job of explaining bit by bit why Hillary Clinton does not have a path to the nomination.
What she has is more like an Odyssey and there's not much in her favor.
She can try to keep moving the goal posts and changing the rules, but if you play around with the delegate numbers, something very interesting happens.
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If things remain even and close
If you assume an even split of delegates through May 6, Obama would have 1571 delegates to Hillary's 1434 (these are rough estimates, which is good enough to make this point). The current Super Delegate split according to MSNBC has Obama at 211 and Clinton at 254.
Add these numbers to post May 6 and you get 1782 and 1688. What's left of the Super Delegates is 331, which is enough to put Obama over 2025 (in fact, he only needs 243 of the remaining Supers). Clinton, on the other hand, would fall short of the 2025 even if she got all the remaining supers.
Of course, between now and then some of these remaining 331 will start to side up, and with Obama out in front and with history as a guide, this split is likely to favor Obama rather than Clinton.
In any event, the point I'm trying to make is that if things remain tight and even through May 6, the race will technically be callable. At this point, Obama will practically be licking the finish line. He'll lead in states, pledged delegates, and the popular vote. Hillary will have an even more dismal outlook than she has now because she failed to gain any significant delegates or a lead in the popular vote.
An analogy: have you ever bet on a horse that is beyond the ability to win in the final stretch and you toss your betting slip away before the race is even over? Meanwhile, the person that bet the horse out in front is already headed to collect the money before the winner even crosses the line.
This race isn't in the final stretch yet. People are all still holding their tickets. But at some point in May people are going to simultaneously toss or cash in their tickets, and the race will be essentially called.