Congress is putting nonprofits under a microscope.

Scandals at such name-brand organizations as the Nature Conservancy, the United Way and American University are spurring an effort on the Hill to crack down on some charitable practices.

This week, senators are expected to propose rules that would limit a number of charitable write-offs.

The proposals are the latest in a series of efforts in recent years by Congress and the Internal Revenue Service to rein in what some legislators view as abusive practices among some of the nation's 1.8 million tax-exempt entities.

Last year, Congress tightened rules for donations of vehicles and gifts of intellectual property.

This week, Senate Finance Committee Chairman Charles E. Grassley (R-Iowa) is expected to propose amendments to the tax-reconciliation bill -- set to go to the Senate floor this month -- that would limit a number of charitable write-offs.

One proposal would tighten rules on appraisals of non-cash charitable donations, such as art, land and closely held stock. Another would limit deductions for donations of easements that protect the outward appearance of historic buildings, Senate aides said this week.

And those who donate clothing and household goods to charities will no longer be able to guess at their value. Under a Grassley proposal, they would have to consult a guide, to be published by the Internal Revenue Service, to determine the value of the items.

"Let me be clear," Grassley told attendees at the annual conference of Independent Sector, a coalition of 500 nonprofit groups, in Washington last month, "we're seeing serious abuses in parts of the charitable sector. I'll take action when and where I can to deal with those abuses."

The news isn't all grim for charities and their contributors. Sen. Rick Santorum (R-Pa.) is expected to offer a proposal allowing taxpayers to withdraw money from their individual retirement accounts and donate it to charities, a provision long sought by nonprofit organizations.

And under a hurricane tax-relief law enacted in September, individuals can write off cash contributions equal to up to 100 percent of their adjusted gross income, compared with the usual limit of 50 percent of income. But those contributions need to be made between Aug. 28 and Dec. 31, 2005.

Another provision in the hurricane tax-relief law gives a bit of a tax break to Hurricane Katrina volunteers, allowing them to write off 34 cents a mile if they use their own cars for Hurricane Katrina relief efforts, instead of the current 14 cents per mile.

Congress isn't done yet, however. In the coming months, it is expected to consider proposals that would expand disclosure requirements for charities and modify the way they govern themselves.