Its not just that she scraped home a small win whilst Obama won decisively. Her problem is that Indiana is by any standards a firewall state for her with strongly favorable demographics - she shouldve pulled a big one there and she shouldve at least come close to Obama in North Carolina.
Whats worse for her...it is now beyond the realms of possibilities that even landslide victories in all the remaining primaries would give her a lead (which is extremely unlikely too - she will win big in Kentucky and a couple of other places, but Obama has a couple of probable wins coming too).
Furthermore, she has been banging on about the popular vote being somehow and suddenly important. Which contradicts her campaign manager who said in january that the popular vote didnt matter "this is a game about delegates folks". But that aside, if it is about the popular vote - Obama has been ahead in the popular vote for month, he expanded his lead by no less than 150.000 votes yesterday...and its basically impossible for her to catch him now.
There has then been talks that she will "go nuclear"...and use her popularity with party elders on the convention board to forcible seat delegates from Florida and Michigan. Now, she was the only one campaigning in Florida and the only democrat on the ballot in Michigan - it would basically have no legitimacy. Furthermore, it wouldnt be enough to give her a lead in either the popular vote or delegates.
Clinton has hinted that this isnt a problem, as she believes she can sway enough pledged Obama delegates to vote against the mandate given to them by voters....now, even if that was so, even if the superdelegates didnt intervene to stop the madness...how does she think that will play in the media, with voters who suddenly had their "vote stolen" ? Its nothing short of a "mutual destruction" scheme that might indeed stop Obama becoming president...but it sure as hell stops her too. The only one smiling would be McCain.
Luckily, its unlikely to come to that. After the blows she took in NC and Indiana shes almost certainly gonna face heaps of superdelegates swinging towards Obama - the less significant undecided ones will start jumping on the Obama train in the next few days, a few superdelegates who has already endorsed her will do a "Its over. Please pull out Hillary. Were are changing our endorsement to Obama"...and its gonna be another nail in the coffin every single time, causing bad publicity, a negative campaign atmosphere, a sense round her voters of being on the losing team etc etc...Basically, it is over. Its Bye Bye Hillary.