Is Kerry Paying His "Fair Share"?

Stephen Moore, president of the Club for Growth, says no:

According to the Kerrys' own tax records, and they have not released all of them, the couple had a combined income of $6.8 million in income last year and paid $725,000 in income taxes. That means their effective tax rate was a whopping 12.8%....

Under the current tax system the middle class pays far more than the Kerry tax rate. In fact, the average federal tax rate -- combined payroll and income tax -- for a middle-class family is closer to 20% or more. George W. and Laura Bush, who had an income one- tenth of the Kerrys', paid a tax rate of 30%. ...

Here is the man who finds clever ways to reduce his own tax liability while voting for higher taxes on the middle class dozens of times in his Senate career. He even voted against the Bush tax cut that saves each middle-class family about $1,000.

The Kerrys have unwittingly made the case for what George W. Bush says he wants to do: radically simplify and flatten out the tax code. ... So before John Kerry is given the opportunity to raise taxes again on American workers, shouldn't he and Teresa at least pay their fair share?

Here's what I've never understood: why don't those who feel they're not paying their "fair share" voluntarily do so?