The Congressional Budget Office has released Options for Changing the Tax Treatment of Charitable Giving:

The deductibility of charitable donations has been a feature of the U.S. individual income tax almost as long as the modern income tax has been in existence. Notwithstanding the long duration of that deduction, concerns about its cost, equity, and efficiency have prompted many proposals to change the tax treatment of charitable contributions. At the request of the former Chairman of the House Committee on the Budget, the CBO has examined patterns of individual charitable giving and analyzed how options for changing the tax treatment of such giving might affect the overall level of donations, the costs to the federal government, and the distribution of tax benefits by income group.

Total Charitable Contributions by Individual Donors, 1963 to 2009

 
 
Percentage of Income That Tax Filers Contribute to Charity, by Income Group, 2008