Actually more Americans than not also give W the credit for our current economy.
I also don't blame GOP'ers for not caring if Mitt is lacking substance...
If your concern is truly over "lacking substance," poll results are irrelevant. I'm sure we can both think of candidates, on either side, who've enjoyed public support for positions that weren't substantive.
Furthermore, falling back on a poll only undercuts your point, insofar as it seems to indicate that arguments lacking substance can be effective.
