Sen. Charles E. Grassley, senior Republican on the Senate Finance Committee, is asking the Council on Foundations for information about the need for a proposed change in the way foundations pay excise tax on their net investment income.

Supporters of the proposal, which Mr. Grassley said is pending in the Senate as an amendment to a tax bill, say such a change would simplify the tax code for foundations and encourage grant makers to give more during the current economic hard times.

Foundations currently are subject to a 2-percent or a 1-percent tax. They can qualify for the lower rate in any year in which the percentage of assets they directed toward charitable distributions is larger than the average percentage of their distributions during the previous five years.

While the two-tier tax was intended as an incentive for foundations to give more, some lawmakers and foundation officials have argued it has had the opposite effect. They say it pushes foundations not to raise their grant making significantly in any one year because that increase would raise the average donation amount, thereby requiring the organizations to continue to give at a higher rate in subsequent years to avoid the 2-percent tax rate.

The proposed change would eliminate the current two-tier excise-tax system and replace it with a flat rate of less than 2 percent.

In a letter to the Council on Foundations, which supports the change, Mr. Grassley, of Iowa, said he supports the concept of simplifying the excise tax but thinks any change should be considered in the context of “larger reform” with hearings and formal study.

What’s more, he said, “while some claim that simplifying and flattening the rate will increase payout, it is not clear to me that will be the actual result.”

Mr. Grassley asked the Council on Foundations for background information, including how many private foundations actually pay the investment-income excise tax.