Need to distinguish left vs. right across three grids, imo: spending and taxation, social policy, national security.
If you did so, my sense is that Obama is very left on spending and tax, somewhat left on social policy (but not that much further than Bush), and in the center or even to the right of center on national security.
In comparison, put Clinton in the middle on spending and taxation, further left on social policy than Obama, and a little further left on national security than Obama.
Bush, put much further right than either of the others on spending and taxation, left of center on social policy, and further right than the others on national security.
Right now, I think the country is focused on spending and taxation. If Obama succeeds in either outright transforming, or in appearing to transform, to the right on spending and taxation I think he wins easily in '12. His views on social policy and on national security are not a problem for most Americans.
Someone like Palin would be much further right on social policy, in particular, and that is not imo viewed by many people as a justification to vote for her because the country is by and large to the left now on social policy and much more worried about other things.