The Tax Foundation - How Do the Presidential Candidates Tax Plans Affect Taxpayers Marginal Tax Rates? To the surprise of some, even though Senator Obama's tax plan lowers taxes for the bottom four quintiles, marginal tax rates would fall only for the very lowest-income couples. Taking both income and payroll taxes into account, those at the very bottom of the income distribution would see their effective marginal tax rates fall from 27.4 percent to minus 58.6 percent due to proposed changes to the earned income tax credit and Senator Obama's new "Making Work Pay" credit. Most low- and moderate-income couples would see their effective marginal tax rates rise, in some cases, significantly. Indeed, some low- and moderate-income taxpayers will see their marginal rates rise to more than 50 percent. High-income taxpayers can also expect their effective marginal tax rates to riseto 47.2 percent-under Senator Obama's tax plan. This increase is caused by rolling back the 2001 and 2003 reductions in the top two tax rates, curtailing deductions and exemptions at high income levels, and potentially raising Social Security taxes. Senator McCain's tax plan also changes marginal tax rates. His proposal to replace the exclusion for employer-based health insurance with a new health tax credit boosts taxpayers' taxable incomes by their health insurance premiums which generally pushes taxpayers into higher tax brackets, but not to as great an extent as Senator Obama's tax plan.