This election will be decided by about 8% of those undecided centrists who have no ideology and will generally vote for whoever they feel as the best answers to the issues they care about. And what they care about has nothing to do with the power base of either party (they don't care much about hot-button leftist issues, like environment, universal health care, welfare, etc... or abortion, state's rights, stem-cell research bans, assisted suicide, etc...) These undecides are NOT an ethnic minority, they are not working poor, they wealthy business owners or executives, nor do they live in the large urban areas; they are middle to upper middle class heartlanders, mostly college educated, mostly white, many women with TRADITIONALIST (not necessarily conservative or liberal) values. Republican presidential candidates usually overwhelminingly appeal to that group while the hard left loons the dems keep nominating do not. And it usually takes until September of the election year for the polls to start reflecting that.
Obama has to pretty much dump the left wing of his own party for the next two months and run to center-right very hard now to win over this group. McCain is already a moderate republican so obama has some work to do.