For example, The amount one can deduct for charitable contributions can't be more than 50% of one's adjusted gross income. The charitable contribution deduction may be further limited to 30% or 20% of one's AGI, depending on the type of property donated and the type of organization to which it is donated.
I'm not sure how often the above limitations come into play. A more common limit on charitable contributions applies to taxpayers' whose adjusted gross income is more than $311,300 (married filing jointly or qualifying widow(er)); $285,350 (head of household); $259,400 (single); or $155,650 (married filing separately). The formula by which the allowable amount of the deduction for charitable contributions is complex. One example: for a married couple with $426,300 in adjusted gross income and $24,000 in itemized deductions (certain categories excluded) the would have to reduce their itemized deductions by at least $3,375. For a higher income taxpayer (a George Soros for instance), the phase out of the deduction likely would be much higher (up to 80 percent).
Without seeing the details of the new proposal, it's difficult to see how it would impact the George Soros' of the world beyond the existing limits. What would worry me is that it won't really be aimed at higher-end taxpayers, who already give large donations often without that much consideration to the tax savings (which as noted are limited), but at "regular" taxpayers who still get to take the full amount of their donations as a deduction.