Economy?
More specifically, decent jobs (with salaries that take into account the unreported inflation).
That is Obama’s weakest spot; he wants to create a nation of government-dependent serfs, while Repubs need to create more workers via individual and corporate tax cuts (who then became taxpayers, parents, maybe homeowners). The issues in all three sectors (falling tax revenues, declining American birthrates, and the housing market) can all be attributed to a lack of stable, decent jobs.
Without jobs we’ve become Western Europeans...