This is why the nomination race really and truly is over, barring a complete meltdown by Barack Obama:
Top Democratic officials said privately that Congressional leaders were content to have the race play out as long as it did not take on a negative tone. Attacks on Mr. Obama by the Clinton campaign or its surrogates could lead to a leadership push for superdelegates to show their hand and bring the race to a close, said aides, who did not want be identified discussing internal strategy.The closest that Clinton can come to a plausible scenario for winning the nomination is for a number of things to occur for her at once. She wins the rules fight over Florida and Michigan and she finds a new line of attack against Barack Obama that proves effective in Kentucky and Oregon and she is able to persuade enough superdelegates to endorse her (or at least hold off on endorsing Obama) to give herself a fighting chance.
The problem is that, as the New York Times piece points out, these things do not behave independently from one another. If Clinton goes too negative on Barack Obama, or advances arguments that are perceived as undermining his legitimacy, she might lose two superdelegates for every elected delegate that she would gain. Thus, while she can continue to campaign to her heart's content, she is handcuffed in terms of her strategic options.
Likewise, if Clinton goes "nuclear" on Florida and Michigan and is perceived as having run an end-around through the Rules Committee, the superdelegates will not be pleased, and may move toward Obama in enough numbers to offset many of her gains. Thus, the role of the superdelegates has become that of a thermostat, ensuring the campaign ends neither too hot nor too cold.