Wall Street Journal op-ed:  A Christmas Wish for Charities, by Kimberly O. Dennis (President & CEO, Searle Freedom Trust):

Nonprofits that oppose a limit on deductions but agree to higher tax rates make a devil's bargain.

Data from Giving USA, which tracks trends in American philanthropy, show
that charitable donations as a percentage of disposable personal income
has remained right around the 2% mark for decades.

It was 2% of disposable income in 1974, when the top marginal tax rate
was 70%, and it was 2% in 1989, when the highest rate was 28%. Last year
it was 1.9%. This suggests that the way to increase giving is to
increase the amount of disposable income people have, and one way to do
that would be to lower their taxes. To the extent that higher tax rates
reduce disposable income, they will likely only depress giving.

Other Giving USA data going back to 1971 show giving to be remarkably
constant as a share of gross domestic product. This figure, too, has
remained relatively constant at around 2%, never going below 1.7% or
above 2.3%. In 1971, giving was 2.1% of GDP; last year it was 2%. If the
goal is to increase giving, the focus should be on raising GDP, not
taxes. …

If nonprofits knew what was good for them, they would be focusing less
on preserving the charitable deduction and more on economic growth and
wealth creation. As the dreaded fiscal cliff approaches, they should be
lobbying for a tax system that lowers rates and eliminates loopholes,
allowing capital—including charitable capital—to flow to its most
productive use. The resulting prosperity would do much more for
charities than preserving their own special carve-out from a punitive
tax structure.