That is a good point about the interaction between the $50,000 exemption and the Social Security trust fund, which is receives revenues from the taxation of Social Security benefits. (The Medicare trust fund also receives a portion of the tax paid by higher-income seniors.) I don't think Senator Obama's payroll tax exemption would affect the trust fund, however, since it is implement as a refundable 6.2 percent tax credit. That is, it would reduce income tax, but not payroll tax, revenues.