Mr. Grassley, in requesting an investigation, said that the official’s statement implied “direct knowledge of Koch’s legal and tax status,” in possible violation of taxpayers’ privacy laws, and may have been “politically motivated.” The White House, in a statement, denied any improper accessing of confidential taxpayer information. “The official’s statement was not based on any review of tax filings and we will not use this example in the future,” the White House said.

As the November elections approach, Democrats have complained that well-financed conservative groups, including the Chamber of Commerce, Crossroads GPS, Americans for Prosperity, and Americans for Job Security  have improperly exploited gaps in federal tax and campaign finance law to finance attacks on Democratic candidates.

The Chamber of Commerce, for instance, was accused in a complaint filed last month by a liberal group of improperly funneling money for political work through its nonprofit arm  an assertion the chamber denies. Liberal groups are also questioning whether the chamber has allowed money from foreign corporations to be improperly used in the group’s political efforts. The chamber is spending at least $50 million this election season to promote candidates who have largely opposed increased business regulations.

The liberal blog Think Progress suggested this week that because the Chamber of Commerce’s general fund includes membership money from some foreign corporations, that money could be used in political activities in possible violation of a ban on foreign involvement in campaigns. While liberal groups like Move On quickly seized on the accusation, chamber officials said it did not use any foreign money to finance political activities, and they denounced the report as an erroneous and “desperate attempt to silence those who support free enterprise.”

Senator Baucus, the Democrat who leads the Finance Committee, asked the I.R.S. last week to conduct a broad review into “major” tax-exempt organizations to determine if any were misusing their tax-exempt status. Tax-exempt groups are banned from engaging in politics as their “primary” activity.

Mr. Baucus said “political campaigns and powerful individuals should not be able to use tax-exempt organizations as political pawns to serve their own special interests.”

But Senators Hatch and Kyl, in their own letter to the agency on Wednesday, said: “I.R.S. audits and investigations are specifically intended to be separated from the political process. We expect the I.R.S. will adhere to those standards despite requests to the contrary from high-level political officials.”