I think we need to begin asking whether Hillary Clinton is attempting to sabotage Barack Obama's chances in 2008 in order to preserve a shot for herself in 2012.
If Obama is the nominee and wins, there will not be different Democratic nominee until 2016, when Obama's VP will likely be a front runner and Hillary Clinton will be 68. She knows that if Obama wins in 2008, she will never be President.
As others have pointed out (e.g. kubla000), the Clinton campaign is now running a negative advertisement that attacks Barack Obama in a manner eerily similar to how McCain has been and will be attacking Obama this fall.
Negative attacks are part of politics and Barack Obama needs to be prepared to deal with such attacks. However, if after Tuesday's elections, Hillary Clinton does not close Obama's pledged delegate advantage to within at least 110 pledged delegates, she needs to exit the race.
If Hillary Clinton fails to do so, we in the progressive community need to be ready to point out that she is intentionally sabotaging the Democratic Party in 2008 in order to give herself an opportunity to run in 2012.
This would simply be unacceptable. Hillary Clinton should not put her personal ambition in front of the very people for whom she claims to be fighting; the people who would be devastated by 4 more years of war, 4 more years of a health care system that prioritizes profits over patients, 4 more years of tax and trade policies meant to enrich the investment-class at the expense of the middle-class, 4 more years of environmental policies that put special interest over our children's interest, in short, 4 years of John McCain.
On March 5th, the message from the progressive community needs to be loud and clear, if Hillary Clinton chooses to sabotage the Democratic Party, she will be joining Joe Lieberman in the eyes of the progressive community, and we will find her her own Ned Lamont in 2012.