Rob Long uses two datapoints -- the stunning decline of Barack Obama's popularity, and the rise of Hillary Clinton's -- as the basis for a timeline of the unthinkable: could Hillary Clinton challenge Obama in 2012?
There's certainly space for a McGovernesque movement here -- I think the important datapoint here is the activist left's fundamental rejection by the White House, borne out in the San Francisco protests which united Tea Partiers and Code Pinkers. Obama has disappointed his base in so many areas, either by failing to keep his promises or by dragging his feet on things they view as moral imperatives -- on health care, on Gitmo, on Afghanistan, on security policy, on DADT, and on any number of other policies. There will be ramifications for that disloyalty.
This is a more serious question than you might think, regardless whether Hillary really does write a bestseller in "Delivering the Goods: What I Learned at the State Department, What I Learned from Main Street." I personally think there will be a challenge, though perhaps not from HRC herself, though her angles -- competence, foreign policy acumen, a call to break the last barrier for womanhood, and vindication on the spoiled promises of hope and change -- are rather appealing.
I honestly think there is an opening here for the return of Howard Dean. A strong ideological progressive who had an opening to gain the nomination in 2004 before his very public gaffe (amazingly, there was no YouTube then), he has little to lose and has already established a national network, and elected a great many Democrats with his broad national strategy. He has also been the most openly critical of any potential 2012 candidate of this administration on nearly all the areas I listed above. Clinton would need to leave the State Department in short order after the midterms, and even then, she would've had more of an opening if Obama had tacked further left on his foreign policy, not right.
The real question at this point ought to be: who does Tim Gill want to give money to? Money makes things happen. ("That's why they call it money."). You have to have someone with a preexisting money source or a motivated base. That is a very short list -- but if unemployment is still at ~10%, and Obama's numbers are still average at best, someone on that list will almost certainly make a challenge.