As Congress and President Obama fight over the Bush tax cuts, a small number of left-leaning rich people have come out in support of paying higher taxes. The most famous are the members of the Responsible Wealth Project, who say they pay too little in taxes and want to address inequality.

They may be an eccentric minority, or (in the view of conservatives) a lunatic fringe. But a Quinnipiac University poll this year showed nearly two-thirds of those with household incomes of more than $250,000 a year support raising their...