G.O.P. Senators Question I.R.S. Scrutiny of Donors

At least five donors to such groups have received letters from the agency stating that their donations may be subject to federal gift taxes, invoking a provision of law long on the books but rarely enforced.

The Republicans, all members of the Senate Finance Committee, which has oversight of the I.R.S., said they were concerned that political considerations might have played a role in the agency’s decision to scrutinize such donations more closely. The senators are seeking the names of any agency staff members involved in that decision, along with internal documents and any correspondence with White House officials.

“This pattern of nonenforcement over a period of nearly three decades, coupled with the troubling issues regarding the adverse impact that enforcement might have on the exercise of constitutionally protected rights, raises important questions regarding the timing of the decision to enforce the gift tax on these contributions,” the senators wrote. “Retroactive enforcement of the gift tax in this highly politicized environment raises legitimate concerns and demands further explanation.”

The letter was signed by Senator Orrin G. Hatch of Utah, the ranking member of the Finance Committee, and five of his colleagues.

Michelle Eldridge, a spokeswoman for the I.R.S., said in a statement that the agency was reviewing the letter and did not have an immediate response. It is unclear which individuals received the I.R.S. letters, or how far-reaching the agency’s efforts have been.

Large nonprofit organizations are expected to play an influential role in the 2012 elections. Organized under section 501(c)(4) of the tax law, they may raise vast amounts of money without publicly disclosing the names of their donors and can engage in some forms of political advocacy, though their primary purpose cannot be political in nature.

Democrats, including President Obama, have criticized such groups as conduits for unregulated campaign money, but have recently begun organizing their own versions to match what they say will be an onslaught from Republican-aligned groups, including several that played a major role in helping Republicans win control of the House last year.

In September, Senator Max Baucus, the Montana Democrat who is chairman of the Finance Committee, sent his own letter to the I.R.S., urging the agency to investigate the tax-exempt groups that were engaged in unlawful political activity.