I think the most powerful explanation of Hillary Clinton's actions is that she is engaged in a winner take all competition with a thin but plausible hope of victory and is working towards that goal. I reject the hypothesis that she is setting herself up for a 2012 run against President McCain after sabotaging Obama's chances. The internal political logic is too convoluted as her base is demographically maxed out in providing her net margins and it will worsen over time for her.
If each campaign was a home construction firm, most campaigns would have built houses suitable for Dennis Kucinich on Jupiter, very low floors of initial bedrock support and fairly low ceilings. The home would be good for gnomes. John Edwards built a home good for the Artic circle with strong walls and a steep slope roof to allow the snow and support to rapidly fall off. Barrack Obama's campaign has built a home suitable for Manute Bol or Yao Ming --- low floors and very high ceilings.
The Clinton campaign has built a very different house as her structural advantages and disadvantages going into the campaign were very different. She had very high floors, but fairly low ceilings, she it was built for dwarfs in a low gravity environment. She knew that she had a solid 30% to 40% of the Democratic primary electorate attached to her hip, but gaining the remaining ten to twenty points to assemble either a majority or a sufficient plurality would be very difficult. She has had little room to grow, as this chart depicting the non-Gore opinion polling trends from Pollster.com shows.
Over eighteen months of campaigning she has added roughly 20% of her initial support to her coalition. And she has not picked up too many supporters from her opponents who dropped out. Obama on the other hand has more than doubled his initial support levels. This dynamic played out as Clinton was running as a quasi-incumbent with significant institutional, name recognition and organizational support. She did a good job of consolidating that support into a real and viable coalition unlike Joe Lieberman in 2004, but she has not been able to grow her base significantly.
In the counterfactual of an Obama nomination and an Obama loss in the general election, even if it is seen that Hillary Clinton was an excellent surrogate and supporter for Obama, she is weaker in June 2010 than she was in June 2006 as the Clinton nostalgia is a bit weaker, the demographics of the Democratic primary electorate are less favorable, the activists will be looking for something different and her fundamental problem remains. She can attract a significant coalition within the Democratic Party but that coalition will almost certainly be matched by an equal or greater anti-Clinton coalition. She knew that going into this cycle that would be the case, and her counter-plan to it was to run against multiple anti-Clintons, have them split the vote, win Iowa and New Hampshire to set up an information cascade in her favor and close out on Super Tuesday against one or more weakened opponents. Not a bad plan but reality intervened.
2012 is an absurd theory when the structure of the Democratic primary universe is less favorable than 2008 for Hillary Clinton.
I was not planning to write more on this subject, but 3,000 hits later, it is evident that people wanted to read some more.