As I mentioned earlier, Clinton supporters think the Democratic nomination was somehow stolen by Senator Obama. To the extent that some Clinton supporters may either stay home and not vote in November for Senator Obama (if he is the nominee), or may vote for Senator McCain (despite his pro-life and other very conservative positions), and may even actively campaign against Senator Obama. Some folks also think Senator Obama and his campaign have been sexist, which fuels their vitriol.
I strongly disagree. IMO, the reason Senator Clinton might lose what appeared to be her nomination for the taking is simple - mismanagement. Despite her claims to be ready and prepared, she was not. He campaign relied on loyalty rather than competence (see Mark Penn, Patti Solis Doyle). Senator Clinton continued to believe - till as late as the last debate pre-Super Tuesday (I will try to find a video link and post it) - that the campaign would be over on Super Tuesday (ST).
On February 6, the race was still tied. Senator Obama's campaign was getting ready for the 10 primaries and caucuses that would lead to the March 4 primaries. But Senator Clinton's campaign was out of cash. The Clinton campaign had not done any groundwork in most of the post-ST February states, and the only place where they tried to compete strongly was Maine, saying the other states were either primaries dominated by African-Americans ("he's a Black candidate, he'll win!") or were activist/liberal-dominated caucuses. However, even in Maine (also a caucus!), Senator Obama won with his campaign's superior organizational skills.
That lack of planning, preparedness, and organization between February 5 and March 4 on the part of the Clinton campaign helped Senator Obama build a 100+ pledged delegate lead in those 10 contests, which has proved a practically insurmountable barrier for Senator Clinton. If she had competed effectively, the difference in pledged delegates today would be much smaller, and close enough to be effectively a tie. And then Senator Clinton could have argued that she was more prepared, ready etc. - and might have won.
So there you have it - don't blame Senator Obama for waging an effective campaign, or accuse sexism (not entirely, anyway), or fault caucuses (everybody knew - or should have known - the rules ahead of the race). Senator Clinton campaigned as a super-competent, highly effective manager which, according to her, was the job of the President. But she failed to demonstrate these abilities with her own campaign (much the same, some might say, as with universal health care in the '90s), and therein lies a tale.