This is more just fun than serious but I thought it might be interesting to do a cursory analysis of a three-way race between Hillary Clinton, Barack Obama and John McCain. This could give us some idea what might happen if Obama pursues an independent bid for the presidency after being denied the Democratic nomination.

Pew Research has come out with a poll on party identification:

Democrats: 36%
Republicans: 27%
None: 37%

Of the 37% who claim no party identification, 15% lean Democratic, 10% lean Republican, and 12% have no leaning either way. For the purposes of analyzing a three-way race, let's first group all the "leaners" with the party they lean toward:

Democrats: 51%
Republicans: 37%
None: 12%

Let's assume for the sake of this analysis that Hillary and Barack roughly split the Democratic vote (51 to 49 which corresponds to their popular vote totals) in a three-way race. Let's also assume that McCain gets all of the Republican vote. Now there will obviously be some cross-over there: some Democrats will vote for McCain and some Republicans for Obama (fewer for Clinton) but let's assume these cancel out.

In most of the exit polls I have seen Obama beats Hillary among independents about 2-to-1. McCain also draws well among independents, so let's assume a 40-40-20 breakdown for Obama, McCain and Hillary with independents.

McCain: 37 + 4.8 = 41.8%
Obama: 26 + 4.8 = 30.8%
Clinton: 25 + 2.4 = 27.4%

Another factor to consider is fundraising. Obama has outraised Clinton roughly two-to-one, and they have both raised much more than McCain. Now, money isn't everything as Mitt Romney proved but it definitely gives some advantage. On the power of advertising alone let's give Obama +2, Clinton +1 and McCain -3 (and rounding the numbers):

McCain: 39%
Obama: 33%
Clinton: 28%

Of course, it gets a whole lot more interesting to start breaking down individual states. Ultimately, McCain would probably win a three-way race but Obama would likely come in second. Over the course of a long campaign that dynamic could work to Hillary's disadvantage pushing her down even further.

This is some pretty wild conjecture though because I still believe Obama has the Democratic nomination all but sewn up.