Effective tax rate. Technically although their tax rate is 35%, via loopholes, deductions, making $ off of carried interest, etc the average million dollar + earner paid an effective tax rate of 16.5% (thereabouts) in 2009.
All that The Buffet Rule is saying is that if your tax rate is 35% and youre going to take advantage of all the deductions and loopholes that are out there (which one is free to do), you should at least pay 25% instead of 16.5%.
I dont see how anybody could argue against that. Logically, at least.