Leading Republicans are suggesting that a senior official in the Obama administration may have improperly accessed the tax records of Koch Industries, an oil company whose owners are major conservative donors.

And the Republicans are also upset about an I.R.S. review requested by Senator Max Baucus, the Montana Democrat who leads the Finance Committee, into the political activities of tax-exempt groups. Such a review threatens to “chill the legitimate exercise of First Amendment rights,” wrote two Republican senators, Orrin G. Hatch of Utah and Jon Kyl of Arizona, in a letter sent to the I.R.S. on Wednesday.

Republicans were quick to point out that the I.R.S. was put under tight restrictions about access to Americans’ tax returns as a result of political shenanigans by the Nixon administration involving tax audits.