There are few things that can unite national tax think tanks. One thing they all agree on, however, is that taxes on capital gains are income taxes. I asked national tax groups today if capital gains taxes are classified as an excise/sales tax or income tax. The responses are starting to come in (will update as received):

• Urban Institute: “Income tax."
   
• Center for American Progress: "Income tax."
   
• Progressive Policy Institute: “Capital gains taxes would be considered a form of income tax: the gains are income generated by an owned asset the same way wages are income generated by labor.”
   
• Tax Foundation: "It is an income tax. Capital gains constitute income and, when taxed, are taxed accordingly.”
   
• IRS: "It is an income tax. More specifically, capital gains are treated as income under the tax code and taxed as such."

It appears a trend is developing.

Additional Information
Income tax on capital gains: Follow the bouncing ball edition
A Capital Gains Tax IS an Income Tax: Irrefutable Proof in About Two Minutes
BAD POLICY: An Income Tax on Capital Gains for Washington
Washington Capital Gains Proposal Not Helped by Analogy to Real Estate Excise Tax