I think it will be practically impossible to prove your point. Because in order to prove your point you will have to prove this things.

1) How much the rich report (again I think the top 30% of income earners is a fair place to start.) This figure is easy to find.

2) How much the rich have sheltered. Don't know how you could prove this if it isn't reported anywhere. Someone estimates are useless here and only an opinion.

3) If the rich are sheltering money what kind of shelters are they? Do they have to pay taxes on the money eventually? In other words are they just tax differed shelters or something else. Again almost impossible to show because it isn't reported anywhere.

4) If you want to show that Bush's administration has compounded this problem you then would have to show not only that the Republicans pushed the bills through enabling these shelters and that Bush signed them into law. But also all of the above.

Pretty tall order. I prefer to believe the tax analyzers from the US Treasury dept.